In this work, we explored the interfacial two-dimensional (2D) physics and significant advancements in the application prospects of MoSSe monolayer when it is combined with a boron pnictide (BP, BAs) monolayer in a van der Waals heterostructure (vdWH) setup. The constructed vdWHs were found to be mechanically and dynamically stable, and they form type-II p–n heterojunctions. Thus, the photogenerated electron–hole pairs are spatially separated. In the BX/MoSSe vdWHs, the BX monolayer serves as excellent donor material for MoSSe, having an ideal donor band gap of ∼1.3 eV. The small value of the conduction band offset (CBO) between the individual monolayers in the vdWHs makes it an excellent candidate for solar energy harvesting in excitonic solar cells, where the power conversion efficiencies were calculated to be 22.97% (BP/MoSSe) and 20.86% (BAs/MoSSe). Also, more than four-fold enhancement in the out-of-plane piezoelectric coefficient (d33) was observed in the MoSSe-based vdWH relative to that in the MoS2-based vdWH owing to the intrinsic built-in vertical electric field in MoSSe. This is consistent with the out-of-plane piezoelectricity brought about by the alteration in symmetry at the metal–semiconductor Schottky junction, which has been observed experimentally [M.-M. Yang, Z.-D. Luo, Z. Mi, J. Zhao, S. P. E and M. Alexe, Nature, 2020, 584, 377–381]. The results obtained in this work provide useful insights into the design of nanomaterials for future applications in nano-optoelectronics, more efficient excitonic solar cells, and nanoelectromechanical systems (NEMS). Furthermore, this work demonstrates outstanding potential for the application of these vdWHs in superfast electronics, including low-power digital data storage and memory devices, where the tunnel current between the source and drain is effectively tunable using a normal electric field of small magnitude serving as the gate voltage.

Interfacial hybridization of Janus MoSSe and BX (X = P, As) monolayers for ultrathin excitonic solar cells, nanopiezotronics and low-power memory devices.

2D HfN2/graphene interface based Schottky device: Unmatched controllability in electrical contacts and carrier concentration via electrostatic gating and out-of-plane strain

Two-dimensional (2D) monolayered magnesium triphosphides (MgP3) is identified as a new member of the 2D XP3 family. Interestingly, a strain less than 8% can induce the bandgap of the MgP3 monolayer changing in a large range from 0.32 to 1.35 eV. On the basis of the first-principles calculations, the geometrical structure of the newfound MgP3 monolayer is relaxed, and the dynamical/thermal stabilities are assured by carrying out calculations of phonon dispersion and ab initio molecular dynamics simulation, respectively. The HSE06 calculations give a direct bandgap of 1.18 eV and enhanced visible-light and UV optical absorptions for the MgP 3 monolayer with strain-free. The carrier mobility is higher than those of the several previously reported XP 3 monolayers and demonstrates a significant difference between the electron and the hole carrier mobilities. Strain engineering can significantly affect both the bandgap and optical absorption. These findings indicate that the MgP3 monolayer could have potential applications of optoelectronic, photovoltaic , and photocatalytic materials or devices.

Two-dimensional MgP3 monolayer with remarkably tunable bandgap and enhanced visible-light and UV optical absorptions

The development of low contact resistance at metal-semiconductor interfaces in next-generation transistors is being prioritized to improve device performance. By using density functional theory, an intrinsic Ohmic contact between a wide band gap semiconductor MgS and semimetal graphene is predicted herewith theoretically. The zero Schottky barriers in graphene/MgS van der Waals heterostructure (vdWH) can facilitate a high charge injection efficiency, whereas Ohmic contact can be induced in graphene/MgSe under small external perturbation. A comprehensive investigation of the modulation in the electronic contact properties is conducted under the application of vertical compressive strain and a perpendicular electric field to understand their role in the transport mechanism. Under vertical compressive strain, a band gap of \ensuremath{\sim}0.62 eV in graphene has been opened up. While under perpendicular electric field, the hole carrier concentration in graphene is found to be increased up to $\ensuremath{\sim}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--2}$. Moreover, this work circumvents the prevalent approaches in inducing an Ohmic contact and addresses solutions to very fundamental challenges in pristine graphene, i.e., band gap opening and its tunability, and modulation of carrier concentration. These interesting properties of vdWHs can open up new avenues for constructing these heterojunctions for multifunctional graphene-based field-effect transistors.

Effective modulation of ohmic contact and carrier concentration in a graphene- Mg X ( X = S , Se ) van der Waals heterojunction with tunable band-gap opening via strain and electric field

Two-dimensional (2D) monolayered magnesium triphosphides (MgP3) is identified as a new member of the 2D XP3 family. Interestingly, a strain less than 8% can induce the bandgap of the MgP3 monolayer changing in a large range from 0.32 to 1.35 eV. On the basis of the first-principles calculations, the geometrical structure of the newfound MgP3 monolayer is relaxed, and the dynamical/thermal stabilities are assured by carrying out calculations of phonon dispersion and ab initio molecular dynamics simulation, respectively. The HSE06 calculations give a direct bandgap of 1.18 eV and enhanced visible-light and UV optical absorptions for the MgP3 monolayer with strain-free. The carrier mobility is higher than those of the several previously reported XP3 monolayers and demonstrates a significant difference between the electron and the hole carrier mobilities. Strain engineering can significantly affect both the bandgap and optical absorption. These findings indicate that the MgP3 monolayer could have potential applications of optoelectronic, photovoltaic, and photocatalytic materials or devices. 

The response of the electronic properties of the HfN2 monolayer to external perturbation, such as strain and electric fields, has been extensively investigated using density functional theory calculations for its device-based applications and photocatalysis. The HfN2 monolayer is found to be a semiconductor showing a direct band gap of 1.44 eV, which is widely tunable by 0.9 eV via application of biaxial strain. Furthermore, the tunability in the band edges of the HfN2 monolayer straddling the water redox potential under a biaxial strain of ±10% makes it suitable for solar energy harvesting via photocatalytic applications over a wide range (0–7) of pH. The band gap can be decreased by 29.8% under a biaxial tensile strain of 10%. Upon incorporation of spin orbit coupling (SOC) a large spin splitting at the conduction band (Δc ∼ 314 meV) and a small splitting at the valence band (Δv ∼ 32 meV) are noted, which is attributable to the orbital composition of the band edges. The spin splitting in the band edges is found to be adjustable via biaxial compressive strain. The strain dependent mechanical properties and stability reveal the ability of the HfN2 monolayer to withstand a large magnitude of strain of up to ±10%, thereby bringing about a giant tunability in its Young modulus (Y) from 66 N m−1 to 283 N m−1, which is gainfully exploitable in flexible electronics. The tunability in Y over such a wide range has not been observed in other 2D materials. Moreover, the HfN2 monolayer undergoes a transition from a semiconducting to a metallic state under the application of a normal electric field or gate voltage of 0.48 V A−1, which may potentially serve as the OFF (semiconducting) and ON (metallic) state in devices. Interestingly, an electric field of such intensity has been realized experimentally using pulsed ac field technology. Such a small gate voltage will greatly lower its power consumption. The electronic origin of this transition from the OFF to the ON state is found to arise from unoccupied NFEG (Nearly Free Electron Gas) states. A HfN2 monolayer based tunnel field effect transistor (t-FET) is proposed herewith as a model device for low-power digital data storage, thereby paving new avenues in flexible electronics and memory devices.

Exceptional mechano-electronic properties in the HfN2 monolayer: a promising candidate in low-power flexible electronics, memory devices and photocatalysis.

The next-generation spintronic device demands the gated control of spin transport across the semiconducting channel through the replacement of the external gate voltage source by the piezo potential, as experimentally demonstrated in Zhu et al. ACS Nano, 2018, 12 (2), 1811-1820. Consequently, a high level of out-of-plane piezoelectricity together with a large Rashba spin splitting is sought after in semiconducting channel materials. Inspired by this experiment, a new hexagonal buckled two-dimensional (2D) semiconductor, ZnTe, and its iso-electronic partner, CdTe, are proposed herewith. These 2D materials show a strong spin-orbit coupling (SOC), which is evidenced by a large Rashba constant of 1.06 and 1.27 eV·A, respectively, in ZnTe and CdTe monolayers. Moreover, these Rashba semiconductors exhibit a giant out-of-plane piezoelectric coefficient (d33) = 88.68 and 172.61 pm/V, and can thereby generate a high piezo potential for gating purposes in spin field-effect transistors (spin-FETs). While the low elastic stiffness implies the mechanical flexibility or stretchability in these monolayers. The Rashba constants are found to be effectively modulated via external perturbations, such as strain and electric field. The wide band gap provides ample room for modulation in its electronic properties via external perturbations. Such scope is severely limited in previously reported narrow band gap Rashba semiconductors. The fascinating results found in this work indicate their great potential for applications in next-generation self-powered flexible-piezo-spintronic devices. Moreover, a new class of hexagonal buckled ZnX (X: S, Se, or Te) monolayers is proposed herein based on their previously synthesized bulk counterparts, while their electronic, mechanical, piezoelectric, and thermal properties have been thoroughly investigated using the state-of-art density functional theory (DFT).

I. S. Fathima

Papers

Exceptional mechano-electronic properties in the HfN2 monolayer: a promising candidate in low-power flexible electronics, memory devices and photocatalysis.

Spin-Current Modulation in Hexagonal Buckled ZnTe and CdTe Monolayers for Self-Powered Flexible-Piezo-Spintronic Devices.