Artificial photosynthesis in group VI B transition metal dichalcogenide monolayers.

Pristine, semiconducting 1T-MX2 (M = Zr, Hf; X = S, Se, Te) monolayers are intrinsically centrosymmetric and non-piezoelectric. This inversion symmetry is broken in their Janus monolayer (non-centrosymmetric) structures, leading to a high degree of piezoelectricity.

A stable ultrathin 2D van der Waals (vdW) heterobilayer, based on the recently synthesized boron monophosphide (BP) and the widely studied molybdenum disulfide (MoS 2 ), has been systematically explored for the conversion of waste heat, solar energy, and nanomechanical energy into electricity. It shows a gigantic figure of merit (ZT) > 12 (4) for p (n)-type doping at 800 K, which is the highest ever reported till date. At room temperature (300 K), ZT reaches 1.1 (0.3) for p (n)-type doping, which is comparable to experimentally measured ZT = 1.1 on the PbTe-PbSnS 2 nanocomposite at 300 K, while it outweighs the Cu 2 Se-CuInSe 2 nanocomposite (ZT = 2.6 at 850 K) and the theoretically calculated ZT = 7 at 600 K on silver halides. Lattice thermal conductivity (κ l ≈ 49 W m −1 K −1 ) calculated at room temperature is lesser than those of black phosphorene (78 W m −1 K −1 ) and arsenene (61 W m −1 K −1 ). The nearly matched lattice constants in the commensurate lattices of the constituent monolayers help to preserve the direct band gap at the K point in the type II vdW heterobilayer of MoS 2 /BP, where BP and MoS 2 serve as donor and acceptor materials, respectively. An ultrahigh carrier mobility of ∼20 × 10 3 cm 2 V −1 s −1 is found, which exceeds those of previously reported transition metal dichalcogenide-based vdW heterostructures. The exciton binding energy (0.5 eV) is close to those of MoS 2 (0.54 eV) and C 3 N 4 (0.33 eV) single layers. The calculated power conversion efficiency (PCE) in the monolayer MoS 2 /BP heterobilayer exceeds 20%. It surpasses the efficiency in MoS 2 / p-Si heterojunction solar cells (5.23%) and competes with the theoretically calculated ones, as listed in the manuscript. Furthermore, a high optical absorbance (∼10 5 cm −1 ) of visible light and a small conduction band offset (0.13 eV) make MoS 2 / BP very promising in 2D excitonic solar cells. The out-of-plane piezoelectric strain coefficient, d 33 ≈ 3.16 pm/V, is found to be enhanced 4-fold (∼14.3 pm/V) upon applying 7% vertical compressive strain on the heterobilayer, which corresponds to ∼1 kbar of hydrostatic pressure. Such a high out-of-plane piezoelectric coefficient, which can tune top-gating effects in ultrathin 2D nanopiezotronics, is a relatively new finding. As BP has been synthesized recently, experimental realization of the multifunctional, versatile MoS 2 /BP heterostructure would be highly feasible.

Using first-principles calculations, we demonstrate a combination of two emergent fields, type II van der Waals heterostructures and Janus structures, for the purpose of optimizing the harvesting of solar and nano-electromechanical energy. The most stable stacking order in these nanoscale heterobilayers comprising Janus monolayers of transition metal dichalcogenides has been ascertained based on the interlayer binding energies. The binding energies in WSeTe/WSTe and MoSeTe/WSTe heterobilayers are found to be −27.93 and −25.67 meV/Å 2 at an equilibrium interlayer layer distance of 3.25 and 3.32 Å, respectively, indicating the exothermicity in the process of heterobilayer formation, and hence, its experimental feasibility. The mechanical and dynamical stabilities have also been confirmed for these heterobilayers using the Born Huang stability criteria and phonon dispersion calculations. Our results unveil the mechanism underlying the electronic, piezoelectric, photocatalytic properties, and carrier mobility in these Janus heterobilayers. The power conversion efficiency in the 2D ultrathin excitonic solar cells constituted by some of the heterobilayers studied in this work has been found to lie in the range of 15−20%. Moreover, a very high carrier mobility (>200 cm 2 /V•s) together with a large visible-light absorption coefficient (α ≈ 10 5 cm −1 ) has been observed in these heterobilayers. The piezoelectric coefficients in these ultrathin heterobilayers (d 33 = 13.91 pm/V) is found to reach close to the values obtained in multilayer/bulk structures built from Janus monolayers of Mo-based dichalcogenides. Our findings highlight the promising applications of these heterobilayers in ultrathin excitonic solar cells, nanoelectronics, and nanopiezotronics.

The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional (2D) materials is unusually scarce. Inversion symmetry occurring in MX3 (M = Ti, Zr, Hf; X = S, Se) monolayers is broken upon constructing their Janus monolayer structures MX2Y (X ≠ Y =S, Se), thereby inducing a large out-of-plane piezoelectric constant d 33 (∼68 pm/V) in them. d 33 can be further enhanced to a super high value of ∼1000 pm/V upon applying vertical compressive strain in the van der Waals bilayers constituted by interfacing these Janus monolayers. Therefore, d 33 in these Janus transition-metal trichalcogenide (TMTC) bilayers reach more than 4-fold times that of bulk ceramic PZT material (∼268 pm/V). The absence of a horizontal mirror symmetry and the presence of strong spin–orbit coupling cause Rashba spin-splitting in electronic bands in these Janus 2D monolayers, which shows up as an ultrahigh Rashba parameter, αR ∼ 1.1 eV Å. It can be raised to 1.41 eV Å via compressive strain. Most of the 2D materials reported to date mainly show in-plane electric polarization, which severely limits their prospects in piezotronic devices. In this present work, the piezoelectricity shown by the Janus monolayers of group IV TMTCs and their bilayers is significantly higher than the ones generally utilized in the form of three-dimensional bulk piezoelectric solids, for example, α-quartz (d 11 = 2.3 pm/V), wurtzite-GaN (d 33 = 3.1 pm/V), and wurtzite-AlN (d 33 = 5.6 pm/V). It is exceedingly higher than that in Janus multilayer/bulk structures of Mo- and W-based transition-metal dichalcogenides, for example, MoSTe (d 33 ∼ 10 pm/V). The 2D Janus TMTC monolayers and their bilayers reported herewith straddle giant Rashba spin-splitting and ultrahigh piezoelectricity, thereby making them immensely promising candidates in the next-generation electronics, piezotronics, spintronics, flexible electronics, and piezoelectric devices.