Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures

First-Principles High-Throughput Inverse Design of Direct Momentum-Matching Band Alignment van der Waals Heterostructures Utilizing Two-Dimensional Indirect Semiconductors

Two-dimensional (2D) van der Waals (vdW) heterostructures have attracted widespread attention in photocatalysis. Herein, we employ a novel strategy utilizing first-principles high-throughput inverse design of 2D Z-scheme heterojunctions for photocatalysis. This approach is anchored in high-throughput screening conditions, which are fundamentally based on the characteristics of carrier mechanisms influenced significantly by Z-scheme heterojunctions. A pivotal element of our screening process is the integration of the indirect-to-direct bandgap transition with momentum-matching band alignment in k-space, guiding us to combine two 2D indirect bandgap monolayers into direct Z-scheme heterojunctions characterized by pronounced interlayer excitons. Various stacking modes introduce extra and distinct degrees of freedom that can be useful for tuning the properties of heterostructures, encompassing factors such as components, stacking patterns, and sequences. We demonstrate that various stacking modes can facilitate the indirect-to-direct bandgap transition and the emergence of interlayer excitons. These findings provide exciting opportunities for designing Z-scheme heterojunctions in photocatalysis.

Nonlinear and Negative Effective Diffusivity of Interlayer Excitons in Moiré-Free Heterobilayers

Interlayer exciton diffusion is studied in atomically reconstructed MoSe_{2}/WSe_{2} heterobilayers with suppressed disorder. Local atomic registry is confirmed by characteristic optical absorption, circularly polarized photoluminescence, and g-factor measurements. Using transient microscopy we observe propagation properties of interlayer excitons that are independent from trapping at moiré- or disorder-induced local potentials. Confirmed by characteristic temperature dependence for free particles, linear diffusion coefficients of interlayer excitons at liquid helium temperature and low excitation densities are almost 1000 times higher than in previous observations. We further show that exciton-exciton repulsion and annihilation contribute nearly equally to nonlinear propagation by disentangling the two processes in the experiment and simulations. Finally, we demonstrate effective shrinking of the light emission area over time across several hundreds of picoseconds at the transition from exciton- to the plasma-dominated regimes. Supported by microscopic calculations for band gap renormalization to identify the Mott threshold, this indicates transient crossing between rapidly expanding, short-lived electron-hole plasma and slower, long-lived exciton populations.

Lateral layered semiconductor multijunctions for novel electronic devices

Layered semiconductors, represented by transition metal dichalcogenides, have attached extensive attention due to their unique and tunable electrical and optical properties. In particular, lateral layered semiconductor multijunctions, including homojunctions, heterojunctions, hybrid junctions and superlattices, present a totally new degree of freedom in research on electronic devices beyond traditional materials and their structures, providing unique opportunities for the development of new structures and operation principle-based high performance devices. However, the advances in this field are limited by the precise synthesis of high-quality junctions and greatly hampered by ambiguous device performance limits. Herein, we review the recent key breakthroughs in the design, synthesis, electronic structure and property modulation of lateral semiconductor multijunctions and focus on their application-specific devices. Specifically, the synthesis methods based on different principles, such as chemical and external source-induced methods, are introduced stepwise for the controllable fabrication of semiconductor multijunctions as the basics of device application. Subsequently, their structure and property modulation are discussed, including control of their electronic structure, exciton dynamics and optical properties before the fabrication of lateral layered semiconductor multijunction devices. Precise property control will potentially result in outstanding device performances, including high-quality diodes and FETs, scalable logic and analog circuits, highly efficient optoelectronic devices, and unique electrochemical devices. Lastly, we focus on several of the most essential but unresolved debates in this field, such as the true advantages of few-layer vs. monolayer multijunctions, how sharp the interface should be for specific functional devices, and the superiority of lateral multijunctions over vertical multijunctions, highlighting the next-phase strategy to enhance the performance potential of lateral multijunction devices.

Stacking-Dependent Interlayer Ferroelectric Coupling and Moiré Domains in a Twisted AgBiP2Se6 Bilayer

Rotation/twisting of bilayers could induce unprecedented new physics due to stacking-dependent electronic properties and interlayer coupling, such as the superconductivity in twisted bilayer graphene, which can find applications in electronics. However, deep understanding at the atomic/electronic levels is limited by the capability of accurate theoretical simulations. Here, from first-principles simulations, we found that the AgBiP₂Se₆ bilayer has stacking-dependent ferroelectric ground states due to interlayer polarization coupling. Interlayer ferroelectric coupling is preferred in an AA-stacked AgBiP₂Se₆ bilayer, but antiferroelectric coupling is preferred in AB- or AC-stacked configurations. The ferroelectric Moiré patterns are thus observed in a twisted AgBiP₂Se₆ bilayer with ferroelectric (antiferroelectric) interlayer couplings in the AA (AB/AC)-stacked areas. Our work for the first time unveils the effects of twisting/rotation on interlayer polarization coupling and provides a real example of ferroelectric Moiré patterns.

MoS2 and Janus (MoSSe) based 2D van der Waals heterostructures: emerging direct Z-scheme photocatalysts

Two-dimensional (2D) materials, viz. transition metal dichalcogenides (TMD) and transition metal oxides (TMO), offer a platform that allows the creation of heterostructures with a variety of properties. The optoelectronic industry has observed an upheaval in the research arena of MoS₂ based van der Waals (vdW) heterostructures (HTSs) and Janus structures. Therefore, interest towards these structures is backed by the ability to select their electronic and optical properties. The present study investigates the photocatalytic abilites of bilayer, MoS₂ and Janus (MoSSe) based vdW HTSs, viz. MoS₂/TMO, MoS₂/TMD, MoSSe/TMO and MoSSe/TMD, by a first-principles based approach under the framework of (hybrid) density functional theory (DFT) and many body perturbation theory (GW approximation). We have considered HfS₂, ZrS₂, TiS₂ and WS₂, and HfO₂, T-SnO₂ and T-PtO₂ from the families of TMDs and TMOs, respectively. The photocatalytic properties of these vdW HTSs are thoroughly investigated and compared with their respective individual monolayers by visualizing their band edge alignments, electron-hole recombination and optical properties. Strikingly, we observe that, despite most of the individual monolayers not performing optimally as photocatalysts, type II band edge alignment is noticed in vdW HTSs and they appear to be efficient photocatalysts via the Z-scheme. Moreover, these vdW HTSs have also shown promising optical responses in the visible region. Finally, electron-hole recombination, H₂O adsorption and hydrogen evolution reaction (HER) results establish that MoSSe/HfS₂, MoSSe/TiS₂, MoS₂/T-SnO₂ and MoSSe/ZrS₂ are probable highly efficient Z-scheme photocatalysts.

Stacking-Engineered Heterostructures in Transition Metal Dichalcogenides

The layer-by-layer assembly of 2D transition metal dichalcogenide monolayer blocks to form a 3D stack, with a precisely chosen sequence/angle, is the newest development for these materials. In this way, one can create "van der Waals heterostructures (HSs)," opening up a new realm of materials engineering and novel devices with designed functionalities. Herein, a detailed systematic review of transition metal dichalcogenide stacking-engineered heterostructures, from controllable fabrication to typical characterization, and stacking-correlated physical behaviors is presented. Furthermore, recent advances in stacking design, such as stacking sequence, twist angles, and moiré superlattice heterojunctions, are also comprehensively summarized. Finally, the remaining challenges and possible strategies for using stacking engineering to tune the properties of 2D materials are also outlined.

Electron Density and Its Relation with Electronic and Optical Properties in 2D Mo/W Dichalcogenides

Two-dimensional MX₂ (M = Mo, W; X = S, Se, Te) homo- and heterostructures have attracted extensive attention in electronics and optoelectronics due to their unique structures and properties. In this work, the layer-dependent electronic and optical properties have been studied by varying layer thickness and stacking order. Based on the quantum theory of atoms in molecules, topological analyses on interatomic interactions of layered MX₂ and WX₂/MoX₂, including bond degree (BD), bond length (BL), and bond angle (BA), have been detailed to probe structure-property relationships. Results show that M-X and X-X bonds are strengthened and weakened in layered MX₂ compared to the counterparts in bulks. X-X and M-Se/Te are weakened at compressive strain while strengthened at tensile strain and are more responsive to the former than the latter. Discordant BD variation of individual parts of WX₂/MoX₂ accounts for exclusively distributed electrons and holes, yielding type-II band offsets. X-X BL correlates positively to binding energy (E_b), while X-X BA correlates negatively to lattice mismatch (lm). The resulting interlayer distance limitation evidences constraint-free lattice of vdW structure. Finally, the connection between microscopic interatomic interaction and macroscopic electromagnetic behavior has been quantified firstly by a cubic equation relating to weighted BD summation and static dielectric constant.

Strain robust spin gapless semiconductors/half-metals in transition metal embedded MoSe2monolayer

The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2monolayer, but also convert the semiconducting MoSe2to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM-MoSe2can effectively be tuned under biaxial strain by controlling the spin polarization of the 3dorbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM-MoSe2system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.

Controllable Epitaxial Growth of MoSe2 Bilayers with Different Stacking Orders by Reverse-Flow Chemical Vapor Deposition

The stacking order plays a critical role in the electronic and optical properties of two-dimensional materials. It is however of great challenge to achieve large-size and homogeneous bilayer crystals with precisely controlled stacking orders. Here, we demonstrate an optimized chemical vapor deposition strategy to grow MoSe₂ bilayers with controlled AA or AB stacking sequences. Reverse gas flow effectively suppresses the random nucleation centers, leading to uniform growth of the second layer of MoSe₂ on the first monolayer. A customized temperature profile selectively activates the growth of the MoSe₂ bilayer with different stacking orders: the AA stacking MoSe₂ bilayer tends to form at 810 °C, and the AB stacking MoSe₂ bilayer prefers to grow at a higher temperature of 860 °C. A series of characterization methods confirm that MoSe₂ bilayers with different stacking orders exhibit distinct crystal structures and physical properties. Our results demonstrate a robust and effective route for the controllable synthesis of transition metal dichalcogenide bilayers, which will benefit the development of two-dimensional materials and van der Waals heterostructures.

Predicted Optimal Bifunctional Electrocatalysts for the Hydrogen Evolution Reaction and the Oxygen Evolution Reaction Using Chalcogenide Heterostructures Based on Machine Learning Analysis of in Silico Quantum Mechanics Based High Throughput Screening

Two-dimensional van der Waals heterostructure materials, particularly transition metal dichalcogenides (TMDC), have proved to be excellent photoabsorbers for solar radiation, but performance for such electrocatalysis processes as water splitting to form H₂ and O₂ is not adequate. We propose that dramatically improved performance may be achieved by combining two independent TMDC while optimizing such descriptors as rotational angle, bond length, distance between layers, and the ratio of the bandgaps of two component materials. In this paper we apply the least absolute shrinkage and selection operator (LASSO) process of artificial intelligence incorporating these descriptors together with quantum mechanics (density functional theory) to predict novel structures with predicted superior performance. Our predicted best system is MoTe₂/WTe₂ with a rotation of 300°, which is predicted to have an overpotential of 0.03 V for HER and 0.17 V for OER, dramatically improved over current electrocatalysts for water splitting.

Synthesis of 2D MoS2(1−x)Se2x semiconductor alloy by chemical vapor deposition

Alloying/doping in two-dimensional (2D) materials is emerging as an increasingly important strategy due to the wide-range bandgap tunability and versatility of these materials. Monolayer 2D transition metal dichalcogenide (TMD) alloy has been investigated both theoretically and experimentally in recent years. Here, we synthesized a bilayer MoS_2(1−x)Se_2x semiconductor alloy via the chemical-vapor deposition technique. The as-grown triangular MoS_2(1−x)Se_2x flakes with size of roughly 10 μm were observed by optical microscope and scanning electron microscope (SEM). The 1.4–1.9 nm thickness of the samples, as measured by AFM, means that bilayer MoS_2(1−x)Se_2x alloys were grown. The characteristic Raman modes related to Mo–S and Mo–Se vibrations were observed in the Raman spectrum. Two emission peaks were respectively found, corresponding to the A and B excitons in the photoluminescence (PL) spectrum. XPS measurements confirmed the Se doping of the alloy. The first-principles calculation results show a contraction of the band gap value with the increase of Se doping in the MoS₂ lattice. Compared with monolayer MoS_2(1−x)Se_2x alloy, the band bending effect is more obvious, and the bilayer MoS_2(1−x)Se_2x alloy still shows the direct band gap luminescence characteristic, which has certain guiding significance for the growth of two-dimensional materials and for device preparation.

Alloying/doping in two-dimensional (2D) materials is emerging as an increasingly important strategy due to the wide-range bandgap tunability and versatility of these materials.

A DFT study of Janus structure of S and Se in HfSSe layered as a promising candidate for electronic devices

The transition-metal dichalcogenides are presently being intensively researched because of their unique optoelectronic properties. Further, the success of TMDs in all areas of science without exception has opened the street to find other two-dimensional materials. Based on density functional theory, we study the vibrational and electronic properties of the mixed-phase of S and Se in HfSSe system, i.e, HfSSe monolayer and HfSSe/HfSSe bilayer. In this framework, our systems are full dynamically stable, which shown by their phonon dispersion. Also, we found that the HfSSe (heterolayer) monolayer is an indirect semiconductor (0.63 eV with BLYP), while the HfSSe (alternating) monolayer is a direct semiconductor (0.753 eV with BLYP). Our AA, AB, AA', and AB' of HfSSe/HfSSe (heterolayer) bilayer are indirect band gaps in a range 0.361-0.830 eV, which are promising candidates for electronic devices as field-effect transistors, photodetectors, and other optoelectronics. Nevertheless, HfSSe/HfSSe (alternating) bilayer is a direct bandgap semiconductor with a value of 0.671 eV, when vdW is used.

Direct/indirect band gap tunability in van der Waals heterojunctions based on ternary 2D materials Mo1-x W x Y2

Artificial van der Waals (vdW) heterojunctions assembled by atomically-thin two-dimensional (2D) materials have demonstrated new physical phenomena and unusual properties, thus triggering new electronic, optoelectronic, valleytronic and photocatalytic application. Herein, the electronic band structures of different vdW heterojunctions based on ternary Mo_1-x W _x Y₂ (Y  =  S, Se; x  =  0-1) monolayer with five stacking orders (AA, AA[Formula: see text], A[Formula: see text]B, AB, AB[Formula: see text]) have been investigated using first principle calculations. The direct/indirect band gap has been obtained in the AA[Formula: see text] stacking type-II heterojunctions, ranging from 0.538 eV to 1.260 eV, that are determined by the interlayer distances and stoichiometries. The estimated power conversion efficiency of the AA[Formula: see text] stacking type-II heterojunction varied from 9.1% to 23.4%. The type-I heterojunctions have also been predicted when semiconducting 2H-MoTe₂ monolayer stacks with the specific Mo_1-x W _x Se₂ monolayer, which are MoTe₂/Mo_0.25W_0.75Se₂ and MoTe₂/Mo_0.5W_0.5Se₂. The reported theoretical results can provide broader 2D materials design possibility for the functional devices.

Enhanced sensitivity of MoSe2 monolayer for gas adsorption induced by electric field

According to recent studies, gas sensors based on MoSe₂ have better detection performance than graphene-based sensors, especially for N-based gas molecules, but the reason for that is not fully understood at the microscopic level. Here, we investigate the adsorption of CO, CO₂, NH₃, NO and NO₂ gas molecules on MoSe₂ monolayer by the density functional theory calculations. Our results reveal that indeed MoSe₂ monolayer is more sensitive to adsorption of N-containing gas molecules than C-containing, which can be attributed to the distinct charge transfer between the gas molecules and MoSe₂. The conductance was further calculated using the nonequilibrium Green's function (NEGF) formalism. The reduced conductance was found for NH₃ and NO₂ adsorbed MoSe₂, consistent with the high sensitivity of MoSe₂ for NH₃ and NO₂ molecules in the recent experiments. In addition, the adsorption sensitivity can significantly be improved by an external electric field, which implies the controllable gas detection by MoSe₂. The magnetic moments of adsorbed NO and NO₂ molecules can also be effectively modulated by the field-sensitive charge transfer. Our results not only give microscopic explanations to the recent experiments, but also suggest using MoSe₂ as a promising material for controlled gas sensing.

Reversible direct-indirect band transition in alloying TMDs heterostructures via band engineering

Alloying is a feasible and practical strategy to tune the electronic properties of 2D layered semiconductors. Here, based on first-principles calculations and analysis, we demonstrate band engineering through alloying W into a prototype MoS₂/MoSe₂ heterostructure. Especially, when the W compositions x  >  0.57 in Mo_1-x W _x S₂/MoSe₂, it exhibits remarkable and reversible direct- to indirect-gap transition. This is because for Mo_1-x W _x S₂/MoSe₂, the valence band maximum located at the K point originates from dominant MoSe₂, while the competing Γ state stems from the hybridization of both Mo_1-xW _x S₂ and MoSe₂, which is extremely sensitive to the interlayer coupling. Consequently, alloying in MoS₂ layer induces direct- to indirect-gap transition and gap increase due to the weakened p-d coupling. We also observe that whether initial alloying in MoS₂ or MoSe₂, the µMo-µW poor condition should always be used. Our findings are generally applicable and will significantly expand the band engineering to other alloying TMDs heterostructures.

Piezoelectricity in WSe2/MoS2 heterostructure atomic layers

A two-dimensional heterostructure of WSe2/MoS2 atomic layers has unique piezoelectric characteristics which depend on the number of atomic layers, stacking type and interlayer interaction size. The van der Waals heterostructure of p- and n-type TMDC atomic layers with different work functions forms a type-II staggered gap alignment. The large band offset of the conduction band minimum and the valence band maximum between p-type WSe2 and n-type MoS2 atomic layers leads to large electric polarization and piezoelectricity. The output voltages for a MoS2/WSe2 partial vertical heterostructure with a size of 3.0 nm × 1.5 nm were 0.137 V and 0.183 V under 4% and 8% tensile strains, respectively. The output voltage of an AB-stacking MoS2/WSe2 heterostructure was larger than that of an AA-stacking heterostructure under 4% tensile strain due to the contribution of intrinsic piezoelectricity and symmetric out-of-plane conditions. The AB-stacking has a lower formation energy and better structural stability compared to AA-stacking. The large output voltage of nanoscale partial or full vertical heterostructures of 2D WSe2/MoS2 atomic layers in addition to the increased output voltage through the series connection of multiple nanoscale piezoelectric devices will enable the realization of nano-electromechanical systems (NEMS) with TMDC heterostructure atomic layers.

Step-like band alignment and stacking-dependent band splitting in trilayer TMD heterostructures

We propose a kind of trilayer TMD heterostructure with step-like band alignment, and the effects of interlayer coupling, strain and SOC are also discussed.

Quaternary 2D Transition Metal Dichalcogenides (TMDs) with Tunable Bandgap

Alloying/doping in 2D material is important due to wide range bandgap tunability. Increasing the number of components would increase the degree of freedom which can provide more flexibility in tuning the bandgap and also reduces the growth temperature. Here, synthesis of quaternary alloys Mo_x W_1-x S_2y Se_2(1-y) is reported using chemical vapor deposition. The composition of alloys is tuned by changing the growth temperatures. As a result, the bandgap can be tuned which varies from 1.61 to 1.85 eV. The detailed theoretical calculation supports the experimental observation and shows a possibility of wide tunability of bandgap.