A new family of two-dimensional ferroelastic semiconductors with negative Poisson's ratios

Spatially controllable and mechanically switchable isomorphous organoferroeleastic crystal optical waveguides and networks

The precise, reversible, and diffusionless shape-switching ability of organic ferroelastic crystals, while maintaining their structural integrity, positions them as promising materials for next-generation hybrid photonic devices. Herein, we present versatile bi-directional ferroelasticity and optical waveguide properties of three isomorphous, halogen-based, Schiff base organic crystals. These crystals exhibit sharp bending at multiple interfaces driven by molecular movement around the CH = N bond and subsequent 180° rotational twinning, offering controlled light path manipulation. The ferroelastic nature of these crystals allowed the construction of robust hybrid photonic structures, including Z-shaped configurations, closed-loop networks, and staircase-like hybrid optical waveguides. This study highlights the potential of shape-switchable organoferroelastic crystals as waveguides for applications in programmable photonic devices.

Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y2Se2BrF Monolayer

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y2Se2XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 104 cm2 V-1 s-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y2Se2BrF possesses outstanding d33 (d33 = -405.97 pm/V), greatly outperforming CuInP2S6 by 4.26 times. Overall, the nano Y2Se2BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.

Rational design of 2D Janus P3m1 M2N3 (M = Cu, Zr, and Hf) and their surface-functionalized derivatives: ferromagnetic, piezoelectric, and photocatalytic properties

In this study, first-principles calculations were employed to rationally design two-dimensional (2D) Janus transition metal nitrides of P3m1 M2N3 phases, where M is a d-block element (Sc-Zn, Y-Cd, Hf-Hg). Among the 29 examined 2D M2N3, three 2D phases, namely P3m1 Cu2N3, Zr2N3, and Hf2N3, exhibit excellent thermodynamic, dynamic, mechanical, and thermal stabilities. These novel Janus 2D materials exhibit ferromagnetic metallic and half-metallic behavior. The related 2D Janus surface-functionalized derivatives, Cu2N3H, Cu2N3F, Cu2N3Cl, Zr2N3H, Hf2N3H, and Hf2N3F, are all dynamically stable. The 2D Janus P3m1 phases of Zr2N3H, Hf2N3H, and Hf2N3F, all with M in the +IV oxidation state, act as semiconductors in the visible region, with energy band gaps of 2.26-2.70 eV at the HSE06 level of theory. On the other hand, the 2D Janus P3m1 Cu2N3X phases (where X = H, F, and Cl) are ferromagnetic half-metals. Additionally, it has been unveiled that there are high hole mobilities (∼6 × 103 cm2 V-1 s-1) derived from the moderate deformation potential and effective mass in the 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases. Uniaxial strain engineering has demonstrated the outstanding in-plane piezoelectric properties of 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F with high d11 values (∼99.91 pm V-1). Furthermore, the desirable band-edge alignments and high anisotropic carrier mobilities of 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases indicate their potential as visible light-driven photocatalysts for water splitting reactions on different facets. These properties render 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases promising for use in optoelectronics, piezoelectric sensing, and photocatalysis applications.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Ferroelasticity in Two-Dimensional Tetragonal Materials

Ferroelasticity is a prominent material property analogous to ferroelectricity and ferromagnetism, but its characteristic spontaneous structural polarization has remained less studied and poorly understood. Here, we use a high-throughput computation approach in conjunction with first-principles calculations to identify 65 (M=transition metal, X=nonmetal) monolayers exhibiting in-plane ferroelasticity out of 166 stable tetragonal monolayers. Molecular orbital theory analysis reveals that ferroelastic distortion arises when M-d/X-p and M-d/M-d couplings are both sufficiently weak. We have developed a physically interpretable one-dimensional descriptor that correctly predicts 89% of ferroelastics or nonferroelastics among the examined MX monolayers. Moreover, we find eleven MX compounds that exhibit strongly coupled ferroelasticity and magnetism driven by strain-controlled magnetocrystalline anisotropy, raising the prospects of developing 2D ferroelasticity-based multiferroics.

Carrier doping-induced strong magnetoelastic coupling in 2D lattice

The realization of intertwined ferroelasticity and ferromagnetism in two-dimensional (2D) lattices is of great interest for broad nanoscale applications but still remains a remarkable challenge. Here, we propose an alternative approach to realize the strongly coupled ferromagnetism and ferroelasticity by carrier doping. We demonstrate that prototypical 2D β-PbO is dynamically, thermally and mechanically stable. Under hole doping, 2D β-PbO possesses ferromagnetism and ferroelasticity simultaneously. Moreover, the robustness of ferromagnetic and ferroelastic orders is doping tunable. In particular, 2D β-PbO features an in-plane easy magnetization axis that is coupled with the lattice direction, enabling the ferroelastic manipulation of the spin direction. Furthermore, the efficient ferroelastic control of the anisotropic optical property and spin splitting in 2D β-PbO are also clarified. Our study highlights a new direction for 2D magnetoelastic research and enables the possibility for multifunctional devices.

A multiferroic vanadium phosphide monolayer with ferromagnetic half-metallicity and topological Dirac states

Ferroelasticity, ferromagnetism, half-metallicity, and topological Dirac states are properties highly sought in two-dimensional (2D) materials for advanced device applications. Here, we report first-principles prediction of a dynamically and thermally stable tetragonal vanadium phosphide (t-VP) monolayer that hosts all these desirable properties. This monolayer is substantially ferromagnetic with polarized spins aligned in the in-plane direction via a d-p-d super-exchange coupling mechanism; meanwhile, its tetragonal lattice enables an intrinsic in-plane ferroelasticity with a reversible strain of 23.4%. As a result, the ferroelasticity is strongly coupled with ferromagnetism via spin-orbit coupling to enable deterministic control over the magnetocrystalline anisotropy by an applied elastic strain. More interestingly, this multiferroic t-VP monolayer possesses half-metallicity with an anisotropic, topological Dirac cone residing in the majority-spin channel. We also predict a multiferroic t-CrN monolayer, whose ferromagnetism features a high Curie temperature of up to 478 K but is weakly coupled to its in-plane ferroelasticity. These results suggest a tetragonal 2D lattice as a robust atomic-scale scaffold on the basis of which fascinating electronic and magnetic properties can be rationally created by a suitable combination of chemical elements.

Two-dimensional ferroelasticity and negative Poisson's ratios in monolayer YbX (X = S, Se, Te)

Two-dimensional ferroelastic materials and two-dimensional materials with negative Poisson's ratios have attracted great interest. Here, using first-principles calculations, we reveal monolayer YbX (X = S, Se, Te) materials that harbor both ferroelasticity and negative Poisson's ratios. Indirect wide band gaps of about 3 eV have been found in these three materials. Mechanical analysis reveals that the three materials are flexible and they possess large in-plane negative Poisson's ratios from -0.114 to -0.366. Meanwhile, the ferroelasticity in the monolayer YbX shows moderate energy barriers and strong ferroelastic signals, beneficial for applications in shape memory devices. These intriguing properties make monolayer YbX promising candidate materials for applications in nanoelectronics and nanomechanics.

Two-dimensional XC6-enes (X = Ge, Sn, Pb) with moderate band gaps, biaxial negative Poisson's ratios, and high carrier mobility

Graphene-based analogs and derivatives provide numerous routes to achieve unconventional properties and potential applications. Particularly, two-dimensional (2D) binary materials of group-IV elements are drawing increasing interest. In this work, we proposed the design of three 2D graphene-based materials, namely, XC₆-enes (X = Ge, Sn, or Pb), based on first-principles calculations. These new materials possess intriguing properties superior to graphene, such as biaxial negative Poisson's ratio (NPR), moderate bandgap, and high carrier mobility. These XC₆-enes comprise sp² carbon and sp³ X (X = Ge, Sn, Pb) atoms with hexagonal and pentagonal units by doping graphene with X atoms. The stability and plausibility of these 2D materials are verified from formation energies, phonon spectra, ab initio molecular dynamic simulations, and elastic constants. The incorporation of X atoms leads to highly anisotropic mechanical properties along with NPR due to the unique tetrahedral structure and hat-shaped configuration. In the equilibrium state, all the XC₆-enes are moderate-band-gap semiconductors. The carrier mobilities of the XC₆-enes were highly anisotropic (∼10⁴ cm^-2 V^-1 s^-1 along the [010]-direction). Such outstanding properties make the 2D frameworks promising for application in novel electronic and micromechanical devices.

Novel Braceletlike BiSbX3 (X = S, Se) Monolayers with an In-Plane Negative Poisson's Ratio and Anisotropic Photoelectric Properties

In this work, we predict two novel two-dimensional (2D) auxetic materials, BiSbX₃ (X = S, Se) monolayers, through first-principles calculations. Attributed to their special braceletlike structure, the in-plane negative Poisson's ratio (NPR) of BiSbS₃ and BiSbSe₃ monolayers are as high as -0.25 and -0.26, respectively. The phonon dispersion calculations, ab initio molecular dynamics simulations, and elastic constants calculations demonstrate that these two monolayers possess excellent dynamic, thermal, and mechanical stabilities. The band gap values of BiSbS₃ and BiSbSe₃ calculated at the HSE level by considering the spin-orbit coupling (SOC) effect are 1.68 and 1.20 eV. The anisotropic carrier mobility and superior optical absorption indicate that they may shine in the next generation of electronic and optoelectronic devices. All of these discoveries not only enrich the types of auxetic materials but also provide a structural reference for designing new auxetic materials on the molecular level. Furthermore, they can provide theoretical guidance for future applications of BiSbX₃ (X = S, Se) monolayers in various fields.

Giant negative Poisson's ratio in two-dimensional V-shaped materials

Two-dimensional (2D) auxetic materials with exceptional negative Poisson's ratios (NPR) are drawing increasing interest due to their potential use in medicine, fasteners, tougher composites and many other applications. Improving the auxetic performance of 2D materials is currently crucial. Here, using first-principles calculations, we demonstrated giant in-plane NPRs in MX monolayers (M = Al, Ga, In, Zn, Cd; X = P, As, Sb, S, Se, Te) with a unique V-shaped configuration. Our calculations showed that GaP, GaAs, GaSb, ZnS and ZnTe monolayers exhibit exceptional all-angle in-plane NPRs. Remarkably, the AlP monolayer possesses a giant NPR of -1.779, by far the largest NPR in 2D materials. The NPRs of these MX monolayers are correlated to the highly anisotropic features of the V-shaped geometry. The exotic mechanical properties of the V-shaped MX monolayers provide a new family of 2D auxetic materials, as well as a useful guidance for tuning the NPR of 2D materials.

Half-auxetic effect and ferroelasticity in a two-dimensional monolayer TiSe

Two-dimensional (2D) materials with both auxetic effect and ferroelasticity are rare, however, have great application potential in next generation microelectromechanical and nanoelectronic devices. Here, we report the findings of an extraordinary combination half-auxetic effect and ferroelasticity in a single p2mm-type TiSe monolayer by performing first-principles calculations. The unique half-auxetic effect, namely the material expand laterally under both uniaxial tensile strain, and compressive strain, is reported and explained by considering both the nearest and the next-nearest interactions. The ferroelasticity is stemming from the degeneracy breaking of the3d-orbitals of Ti atoms in a distorted tetrahedron crystal field, or the so-called Jahn-Teller effect. The results provide a guideline for the future design of novel 2D multiple functional materials at the nanoscale.