Theoretical study of piezoelectric and light absorption properties, and carrier mobilities of Janus TiPX (X = F, Cl, and Br) monolayers

Janus TiPX (X = F, Cl, and Br) monolayers were systematically investigated through first-principles calculations. The Janus TiPX monolayers exhibit mechanical and dynamic stability. Two monolayers are indirect bandgap semiconductors, except the TiPBr monolayer, which has the features of a quasi-direct bandgap semiconductor. Biaxial strain can modify the band gap of single layers. The Janus TiPX monolayers have remarkable flexibility and piezoelectric properties. In particular, the TiPF monolayer shows high horizontal (44.18 pm V-1) and vertical piezoelectric coefficients (-3.59 pm V-1). These values exceed those of conventional bulk materials, like GaN (3.1 pm V-1) and α-quartz (2.3 pm V-1). All of the monolayers have absorption coefficients of 105 cm-1 for visible and ultraviolet (UV) light, which are one order of magnitude greater than that of MoSSe. Furthermore, TiPX monolayers have high carrier mobility. Janus TiPX monolayers represent a class of two-dimensional (2D) materials with exceptional properties and multifunctionality, holding significant promise for various applications in piezoelectric sensors, solar cells, and nano-electronic devices.

First-principles prediction of ferroelectric Janus Si2XY (X/Y = S/Se/Te, X ≠ Y) monolayers with negative Poisson's ratios

Nowadays, two-dimensional (2D) materials with Janus structures evoke much attention due to their unique mechanical and electronic properties. In this work, Janus Pma2-Si2XY (X/Y = S/Se/Te, X ≠ Y) ferroelectric monolayers are firstly proposed and systematically investigated by first-principles calculations. These monolayers exhibit remarkable mechanical properties, including small Young's modulus values, negative Poisson's ratios (NPRs) and large critical strains, reflecting their exceptional flexibility and stretchability. More strikingly, the novel structures of Si2STe and Si2SeTe also endow them with in-plane spontaneous polarization (Ps) and low energy barrier for phase transition, with Ps and energy barrier values being 1.632 × 10-10 C m-1 and 159 meV for Si2STe and 1.149 × 10-10 C m-1 and 196.6 meV for Si2SeTe. The ab initio molecular dynamics (AIMD) simulations reveal high Curie temperatures (Tc) for Si2STe and Si2SeTe, ranging between 1300 K and 1400 K. Additionally, Si2XY monolayers exhibit high anisotropic carrier mobility (∼103 cm2 V-1 s-1) and an extraordinary light absorption coefficient (∼105 cm-1). Our research not only broadens the family of 2D Janus ferroelectric materials, but also demonstrates their potential applications in nanomechanical, nanoelectronic and optoelectronic devices.

W4PCl11 monolayer: an unexplored 2D material with moderate direct bandgap and strong visible-light absorption for highly efficient solar cells

The discovery of novel two-dimensional (2D) materials with excellent electronic and optoelectronic properties have attracted much scientific attention. Based on the first-principles calculations, we predict an unexplored 2D W₄PCl₁₁ monolayer, which is potentially strippable from its bulk counterpart with the exfoliation energy of only 0.16 J m^-2. The dynamical, thermal, and mechanical stabilities have also been confirmed. Remarkably, W₄PCl₁₁ monolayer is direct semiconductor with a bandgap of 1.25 eV, which endows the monolayer with very strong visible-light absorption in the magnitude of 10⁵ cm^-1. Meanwhile, the calculated carrier mobilities of W₄PCl₁₁ monolayer can reach to 10³ cm² V^-1 s^-1. Considering the moderate direct bandgap and high carrier mobility, W₄PCl₁₁ monolayer should be a superior candidate for the donor material of excitonic solar cells. The estimated power conversion efficiency of the fabricated W₄PCl₁₁/Bi₂WO₆ heterojunction reaches as high as 21.64%, which much superior to those of most recently reported 2D heterojunction. All these outstanding properties accompanied with its experimental feasibility endows W₄PCl₁₁ monolayer with promising photovoltaic applications.

Monolayer GaOCl: a novel wide-bandgap 2D material with hole-doping-induced ferromagnetism and multidirectional piezoelectricity

Two-dimensional (2D) materials with excellent properties are emerging as promising candidates in electronics and spintronics. In this work, a novel GaOCl monolayer is proposed and studied systematically based on first-principles calculations. With excellent thermal and dynamic stability at room temperature, its wide direct bandgap (4.46 eV) can be further modulated under applied strains. The 2D semiconductor exhibits high mechanical flexibility, and anisotropy in Poisson's ratio and carrier mobilities, endowing it with a broad spectrum of electronic and optoelectronic applications. More importantly, the GaOCl monolayer has spontaneous magnetization induced by hole doping and shows outstanding multidirectional piezoelectricity, which are comparable with those of either magnetic or piezoelectric 2D materials. Our calculations indicate that the GaOCl monolayer with wide bandgaps and tunable piezoelectricity and ferromagnetism could be promising for applications in multifunctional integrated nano-devices with high performance.

Ultrahigh mechanical flexibility induced superior piezoelectricity of InSeBr-type 2D Janus materials

InSeBr-Type monolayers, ternary In(Se,S)(Br,Cl) compounds, are typical two-dimensional (2D) Janus materials and can be exfoliated from their bulk crystals. The structural stability, electronic properties, mechanical flexibility, and intrinsic piezoelectricity of these InSeBr-type 2D Janus monolayers are comprehensively investigated by first-principles calculations. Our calculations show that the stable InSeBr-type monolayers exhibit ultrahigh mechanical flexibility with low Young's moduli. Due to the amazing flexibility of the InSeBr monolayer with an ultra-low Young's modulus of 0.81 N m^-1, the piezoelectric strain coefficient d₁₁ can reach 10³ pm V^-1 orders of magnitude (around 2361-3224 pm V^-1), which is larger than those of reported 2D materials and even superior to those of conventional perovskite bulk materials. Such a superior piezoelectric response of InSeBr-type monolayers could facilitate their practical applications in sensors and energy harvesters.

Mechanical, electronic and optical properties of a novel B2P6 monolayer: ultrahigh carrier mobility and strong optical absorption

Two-dimensional (2D) materials with a moderate bandgap and high carrier mobility are useful for applications in optoelectronics. In this work, we present a systematic investigation of the mechanical, electronic and optical properties of a B₂P₆ monolayer using first-principles calculations. Monolayer B₂P₆ was estimated to be an anisotropic material from direction-dependent in-plane Young's moduli and Poisson's ratios. Also, B₂P₆ exhibits an ultrahigh electron mobility of ∼5888 cm² V^-1 s^-1, showing advantages for application in high-speed optoelectronic devices. More importantly, for the B₂P₆ monolayer, a desirable transformation from an indirect to direct band gap was observed at a biaxial tensile strain of ∼4%. Increasing the biaxial strain reduces the gap and preserves the suitable band edge positions for photocatalytic water splitting in the observed strain range of 1-8%. The decreased gap also enhances the visible light absorption of the B₂P₆ monolayer. These findings indicate that the B₂P₆ monolayer has promising applications in photocatalytic and photovoltaic devices.

Generating large out-of-plane piezoelectric properties of atomically thin MoS2via defect engineering

We calculated the piezoelectric properties of asymmetrically defected MoS₂ using density functional theory. By creating uneven numbers of defects on either side of two-dimensional MoS₂, the out-of-plane centrosymmetry of the charge distribution is clearly broken, and the out-of-plane piezoelectric response is induced. The largest out-of-plane piezoelectric response is associated with the highest defect ratio for MoS₂ to be semiconducting. We calculated the critical defect density of the metal-insulator transition of the asymmetrically defected MoS₂ to be 9.90 × 10¹⁴ cm^-2 and chemical formula MoS_1.22. The d₃₃ of the multilayer of optimally defected MoS₂ is found to be greater than those of AlN and ZnO, and in the same order of magnitude as lead zirconate titanate. All two-dimensional transition metal dichalcogenides can in principle be fabricated as piezoelectric with this approach. The required defect engineering is readily available with various types of ion irradiation or plasma treatment. By controlling the dose of the ion, the defect ratio and hence the piezoelectricity can be tuned. Such asymmetrically defected transition metal dichalcogenides can easily be integrated into two-dimensional transition metal dichalcogenide based devices, which is hard for conventional piezoelectric thin films to rival.

Electronic and photochemical properties of hybrid binary silicon and germanium derived Janus monolayers

The electronic structures and optical properties of a novel class of hybrid binary Janus materials derived from IV-V groups were investigated using first principles calculations. The computational results demonstrated that, except for Ge₂NAs, all the other five structures of M₂XY monolayers (M = Si, Ge; X, Y = N, P, As; X ≠ Y) have excellent thermal and dynamical stabilities. Janus Si₂NP, Si₂NAs, Si₂PAs and Ge₂NP are semiconductors with direct band gaps spanning the range between 0.82 and 2.49 eV. Notably, the hybrid M₂XY materials exhibit highly efficient absorption within the visible light region, which are greatly higher than their pristine MX structures. Janus Si₂PAs and Ge₂PAs possess appropriate band edge alignments that straddle the water redox potentials in the pH range from 0 to 14, making them promising photocatalysts for water splitting under visible light. Our calculations further demonstrate that the catalytic selectivity for the water splitting reaction could be achieved through the hybrid Janus M₂XY, where, for instance, Ge₂NP appears to facilitate only the oxidation, but not the reduction of water under certain conditions. This outcome provides a new route for the design of novel photocatalysts with improved efficiency and selectivity.