Van der Waals heterostructures for spintronics and opto-spintronics.

Adaptive half-metallicity via magnetic proximity in an electrically sensitive 1T’-WTe2/CrBr3 vdW heterostructure

Optically generated excitonic states (excitons and trions) in transition metal dichalcogenides are highly sensitive to the electronic and magnetic properties of the materials underneath. Modulation and control of the excitonic states in a novel van der Waals (vdW) heterostructure of monolayer MoSe2 on double-layered perovskite Mn oxide ((La0.8 Nd0.2 )1.2 Sr1.8 Mn2 O7 ) is demonstrated, wherein the Mn oxide transforms from a paramagnetic insulator to a ferromagnetic metal. A discontinuous change in the exciton photoluminescence intensity via dielectric screening is observed. Further, a relatively high trion intensity is discovered due to the charge transfer from metallic Mn oxide under the Curie temperature. Moreover, the vdW heterostructures with an ultrathin h-BN spacer layer demonstrate enhanced valley splitting and polarization of excitonic states due to the proximity effect of the ferromagnetic spins of Mn oxide. The controllable h-BN thickness in vdW heterostructures reveals a several-nanometer-long scale of charge transfer as well as a magnetic proximity effect. The vdW heterostructure allows modulation and control of the excitonic states via dielectric screening, charge carriers, and magnetic spins.

Using density functional (DFT) theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M2CO2 (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M2CO2 (M = Ti, Zr, Hf) van der Waals (vdW) heterostructures. Optimized lattice constant, bond length, band structure and bandgap values, effective mass of electrons and holes, work function and conduction and valence band edge potentials of BSe and M2CO2 (M = Ti, Zr, Hf) monolayers are in agreement with previously available data. Binding energies, interlayer distance and Ab initio molecular dynamic simulations (AIMD) calculations show that BSe–M2CO2 (M = Ti, Zr, Hf) vdW heterostructures are stable with specific stacking and demonstrate that these heterostructures might be synthesized in the laboratory. The electronic band structure shows that all the studied vdW heterostructures have indirect bandgap nature – with the CBM and VBM at the Γ–K and Γ-point of BZ for BSe–Ti2CO2, respectively; while for BSe–Zr2CO2 and BSe–Hf2CO2 vdW heterostructures the CBM and VBM lie at the K-point and Γ-point of BZ, respectively. Type-II band alignment in BSe–M2CO2 (M = Ti, Zr, Hf) vdW heterostructures prevent the recombination of electron–hole pairs, and hence are crucial for light harvesting and detection. Absorption spectra are investigated to understand the optical behavior of BSe–M2CO2 (M = Ti, Zr, Hf) vdW heterostructures, where the lowest energy transitions are dominated by excitons. Furthermore, BSe–M2CO2 (M = Ti, Zr, Hf) vdW heterostructures are found to be potential photocatalysts for water splitting at pH = 0, and exhibit enhanced optical properties in the visible light zones.

Theoretical study on the electronic structure, optical and photocatalytic properties of type-II As/CdO van der Waals heterostructure

Stacking different two-dimensional (2D) materials to form van der Waals (vdW) heterostructures has been considered as an effective strategy to realize new and exciting properties and innovative device applications in optoelectronics and electronics. In this work, we theoretically investigated the electronic properties of vdW heterostructures combined with different 2D MX materials (M = Ga, Ge, Sn, In; X = As, Se). Interestingly, SnAs/GaSe vdW heterostructure was screened from all the possible MX vdW heterostructures with the direct band gap (1.25 eV) within the visible light region, which perfectly falls into the optimum range for solar cells. Meanwhile, it was demonstrated that SnAs/GaSe vdW heterostructure has a typical type-II band alignment without strong interface hybridization, thus photogenerated electrons and holes could be effectively separated into opposite layers. In addition, the large band offset, small carrier effective mass, and high tunneling probability across the interface (16.24%) also guarantee its superiority in solar energy conversion. Our results suggest that SnAs/GaSe vdW heterostructure can be a new choice for low dimensional optoelectronic devices particularly for solar cells and pave the way for experimental verification in future.

Electronic and optical properties of InSe/CuI van der Waals heterostructure with type-II band alignment: Effects of vertical strain and electric field

Since the successful exfoliation of graphene, two-dimensional (2D) materials have attracted a lot of scientific interest due to their electronic, chemical, and mechanical properties. Due their reduced dimensionality, these 2D materials exhibit superior mechanical and optoelectronic properties when compared to their bulk counterparts. Within the family of 2D materials, the ultrathin transition metal dichalcogenides (TMDs) such as Tungsten diselenide and Molybdenum disulphide have gained significant attention due to their chemical versatility and tunability. Furthermore, it is possible to leverage the distinct characteristic properties of these 2D materials, which are held together by van der Waals forces, by stacking different 2D layers on top of each other resulting in van der Waals (vdW) heterostructures. Due to the absence of feasible methods to effectively deform the crystal structures of these 2D materials and vdW heterostructures, their mechanical properties have not been thoroughly understood. The atomistic simulations can effectively capture the material behavior at the nanoscale level and help us not only not only understand the mechanical properties of these materials but also aid in the development of tailored processes to tune the material properties for the design of novel metamaterials. Using atomistic simulations, we develop the process - property relationships which can guide the direction of experimentation efforts, thereby making the process of discovering and designing new metamaterials efficient. In this work, we have used laser shock nanostraining technique which is a scalable approach to modulate the optomechanical properties of 2D materials and vdW materials for practical semiconductor industry applications. The deformation mechanisms of 2D materials such as graphene, boron nitride (BN) and TMDs such as WSe2 and MoS2 are examined by employing a laser shocking process. We report studies on crystal structure deformation of multilayered WSe2 and monolayer graphene at ultra-high strain rate using laser shock . The laser shocking process generates high pressure at GPa level, causing asymmetric 3D straining in graphene and a novel kinked-like locking structure in multilayered WSe2. The deformation processes and related mechanical behaviors in laser shocked 2D materials are examined using atomistic simulations. Moire heterostructures can be obtained by introducing a twist angle between these 2D layers, which can result into vdW materials with different properties, thereby adding an additional degree of freedom in the process-property design approach. We were able to successfully create a tunable stain profile in 2D materials and vdW heterostructures to modulate the local properties such as friction, and bandgap by controlling the level of laser shock, twist angle between the 2D layers and by applying appropriate laser shock pressure . We thus extend this knowledge to further explore the pathways of strain modulation using a combination of laser shocking process, moire engineering, and strain engineering in 2D materials consisting of graphene, BN, and MoS2 and to develop the process - property relationships in vdW materials. In summary, this research presents a systematic understanding of the effect of laser shocking process on the van der Waals materials and demonstrates the modulation of mechanical and opto-electronic property using laser nanostraining approach. This understanding provides us with opportunities for deterministic design of 2D materials with controllable properties for semiconductor and nanoelectronics applications.

Two-dimensional (2D) materials with honeycomb structure such as graphene have attracted much attention due to their potential applications in nanoelectronics devices. [1] Furthermore, 2D van der Waals (vdW) heterostructures can be formed by stacking different types of 2D layered materials through the weak interlayer vdW interaction, and have attracted interest due to unique functionalities which are impossible in homogeneous bulk materials. [2,3] For instance, various graphene and hexagonal boron nitride (h-BN) related vdW heterostructures have been demonstrated and their physical properties have been discussed. On the other hand, a new class of 2D materials composed of traditional group III–V materials have been recently proposed. Lucking et al. predicted that the double-layer honeycomb (DLHC) structure where pair of single layer honeycomb structure forms interlayer bonds can be realized in various group III-V, II–VI, and I–VII materials, and some of these 2D materials exhibit exotic topological properties. [4] It has also been reported that most of group III–V and II–VI thin films are stabilized by forming the DLHC structure when the thickness deceases toward the 2D limit whereas a structure with alternating octagonal and square rings called a haeckelite structure is favorable beyond 3–15 monolayers depending on the constituent elements. [5] These findings suggest the realization of a new class of vdW heterostructures consisting of conventional 2D materials (such as graphene, h-BN, and transition-metal dichalcogenides) and DLHC structure. In this study, we explore new stable structures of vdW heterostructures composed of group III-V compounds on the basis of density functional calculations taking account of vdW interaction.The calculations for superlattices consisting of MoS2/AlAs heterostructure reveal that covalent Al-S bonds are formed between Al atoms of AlAs with zinc blende structure and S atoms of MoS2, which stabilizes zinc blende phase over the DLHC structure. This indicates that the combination of transition-metal dichalcogenides and group III-V compounds such as MoS2/AlAs hardly forms vdW heterostructures. On the other hand, we find that vdW heterostructures can be formed for the superlattices consisting of graphene and group III-V compounds (AlAs, AlSb, GaAs, GaSb, InP, InAs, InSb) when its thickness is two monolayers. Furthermore, the vdW heterostructure with DLHC structure in group III-V compounds is more stable than that with conventional zinc blende structure. Therefore, a stable structure whose atomic configurations are different from those of bulk phase is newly found for the combination of graphene and DLHC structure. The calculated biding energies are ranging from 0.22 to 0.30 eV/unit cell, indicating that this vdW heterostructures can be stabilized even at room temperature. The calculations of phonon dispersion also reveal that no imaginary frequencies are present for graphene/InAs with DLHC structure, suggesting its dynamical stability. This is due to nearly coherent in-plane lattice matching (misfit of ~0.1 %) between InAs with DLHC structure and graphene, as demonstrated in InAs nanowire growth by vdW epitaxy. [6] These calculated results suggest that diverse combinations and exotic electronic properties could be discovered in the vdW heterostructures consisting of graphene and group III-V compounds. Acknowledgements: This work was supported in part by the Grant-in-Aid for Scientific Research Grant (JP20K05324, JP19K05268, and JP16H06418) from the JSPS, and KIOXIA Corporation (former Toshiba Memory Corporation).

Vertical stacking of two-dimensional materials into layered van der Waals heterostructures is considered favourable for nanoelectronics and thermoelectric applications. In this work, we investigate the structural, electronic and thermoelectric properties of GeC and Janus monolayers MXO (M = Ti, Zr; X = S, Se) and their van der Waals (vdW) heterostructures using first-principles calculations. The values of binding energies, interlayer distances and thermal stability confirm the stability of these vdW heterostructures. The calculated band structure shows that GeC monolayer have a direct band gap while MXO (M = Ti, Zr; X = S, Se) and their van der Waals heterostructures show indirect band nature. Partial density of states confirms the type-II band alignment of GeC-MXY vdW heterostructures. Our results shows that ZrSeO (GeC) monolayers and GeC-ZrSO vdW heterostructures have higher power factor, making them promising for thermoelectric device applications.

The asymmetrical group III-VI monolayer Janus M2XY (M = Al, Ga, In; X ≠ Y = S, Se, Te) have attracted widespread attention due to their significant optical absorption properties, which are the potential building blocks for van der Waals (vdW) heterostructure solar cells. In this study, we unraveled an In2STe/GeH vdW heterostructure as a candidate for solar cells by screening the Janus M2XY and GeH monolayers on lattice mismatches and electronic band structures based on first-principles calculations. The results highlight that the In2STe/GeH vdW heterostructure exhibits a type-II band gap of 1.25 eV. The optical absorption curve of the In2STe/GeH vdW heterostructure indicates that it possesses significant optical absorption properties in the visible and ultraviolet light areas. In addition, we demonstrate that the In2STe/GeH vdW heterostructure shows high and directionally anisotropic carrier mobility and good stability. Furthermore, strain engineering improves the theoretical power conversion efficiency of the In2STe/GeH vdW heterostructure up to 19.71%. Our present study will provide an idea for designing Janus M2XY and GeH monolayer-based vdW heterostructures for solar cell applications.

Van der Waals (vdW) heterostructures combining 2-D ferromagnets and other nonmagnetic layered materials, such as graphene and WSe2, are highly promising for the realization of novel spintronic devices with integrated magnetic, electronic, and optical functionalities. Among different 2-D ferromagnets, Fe5–xGeTe2 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x~\sim ~0$ </tex-math></inline-formula>) shows a high potential due to its high Curie temperature (TC). Furthermore, compared to commonly used top-down flake stacking strategies, large-scale, all-epitaxial vdW heterostructures are compatible with modern technologies and thus crucial for practical applications. In this contribution, we report on scalable epitaxial growth of Fe5–xGeTe2 via molecular beam epitaxy (MBE) on single-crystalline graphene (on SiC) and WSe2 (on Al2O3) templates. Structural characterizations show the good crystalline quality of the epitaxial Fe5–xGeTe2 films on graphene and WSe2. Importantly, magnetotransport investigations indicate a ferromagnetic order above room temperature for both Fe5–xGeTe2/graphene with perpendicular magnetic anisotropy (PMA) and Fe5–xGeTe2/WSe2 (with low-temperature PMA) heterostructures. Moreover, an unconventional Hall effect (UHE) has been observed in both Fe5–xGeTe2/graphene and Fe5–xGeTe2/WSe2 heterostructures, indicating the formation of topological spin structures. These results represent an important advance regarding bottom-up synthesis of vdW heterostructures beyond conventional exfoliation-based methods, which is crucial for the development of future spintronic applications based on 2-D magnetic materials.

The bilayer heterostructures composed of an ultrathin ferromagnetic metal (FM) and a nonmagnetic material hosting strong spin-orbit coupling (SOC) are a principal resource for spin-orbit torque (SOT) [1] and spin-to-charge conversion [2] effects in next generation spintronics. The key to understand these effect is current-driven nonequilibrium spin density [3]. For example, it generates SOT when it is noncollinear to the direction of local magnetization and it can arise due to variety of microscopic mechanisms, including the spin Hall effect, spin-orbit proximity effect and different interfacial scattering mechanisms. The recently discovered two-dimensional (2D) magnetic materials [4] offer new avenue for highly efficient and gate- or disorder-tunable SOT in van der Waals (vdW) heterostructures composed of few monolayers of atomically thin materials where the spin Hall effect from the bulk is absent. Using first-principles quantum transport calculations, which combine nonequilibrium Green functions with noncollinear density functional theory [1], we predicted [5] that injecting unpolarized charge current parallel to the interface of bilayer-CrI3/monolayer-TaSe2 vdW heterostructure will induce SOT-driven dynamics of magnetization on the first monolayer of CrI3 that is in direct contact with metallic transition metal dichalcogenide (TMD) TaSe2. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we find that this can reverse the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting bilayer CrI3 from antiferromagnet to ferromagnet which can be detetected by passing vertical current and is of potentially great interest to magnetic memory applications since it does not require any external magnetic field. We explain the mechanism of such current-driven nonequilibrium phase transition by showing that first monolayer of CrI3 becomes conducting due to doping by evanescent wavefunctions injected by metallic TaSe2, while concurrently acquiring strong SOC via this proximity effect. Another vdW heterostructure exhibiting SOT is doubly proximitized graphene, which is neither magnetic nor hosts SOC in its isolated form, but proximity induced magnetic moments will exhibit SOT in Cr2Ge2Te6/graphene/WS2 vdW heterostructure which can be tuned by two orders of magnitude via the gate voltage [6]. **

The rapid development of valleytronics makes the application of two-dimensional (2D) transition-metal dichalcogenides (TMDs) in valley electronics important. As a new degree of freedom, valley splitting of TMDs has been achieved and tuned by many methods. Among them, using the magnetic proximity effect (MPE) generated from the interface of 2D van der Waals (vdW) heterostructures stacked with TMDs and one magnetic substrate, valley splitting can be achieved through band edge lifting at the adjacent K/K' valley. However, the comprehensive mechanism and strategy of valley splitting in 2D TMD heterostructures need to be explored ulteriorly. Here, we systematically investigated valley splitting of MX2 in MX2/CrI3 (M = W, Mo; X = S, Se, Te) vdW heterostructures using first-principles approaches. We demonstrated that twisting is an effective method to enhance valley splitting in MX2/CrI3 vdW heterostructures. Furthermore, we also showed a ∼10 times enhancement in valley splitting by changing the stacking patterns between WTe2 and CrI3 layers. We attribute this to the interlayer magnetic and electronic coupling between the two layers of the vdW heterostructure. The present results provide a theoretical basis and effective methods for tuning valley splitting 2D TMD heterostructures.

Low-dimensional materials with prominent thermoelectric (TE) effect play a pivotal role in realizing state-of-the-art nanoscale TE devices. The fusion of TE effect with the magnetism through seamless integration of TE and magnetic materials in the 2D limit offers access to control longitudinal as well as transverse TE properties via magnetic proximity effect. Herein, we design a van der Waals (vdW) heterostructure of metallic 1T-MoS2 with promising TE properties and a layer-dependent magnetic CrI3 material. The result highlights exotic electronic and magnetic configurations of the designed monolayer-CrI3/1T-MoS2 vdW heterostructure, which show magnetically-coupled TE characteristics. The observed remarkable magnetic proximity stems from large magnetic anisotropy energy and spin polarization, which are found to be 2.21 meV Cr−1 and 12.30%, respectively. To this end, the semiconducting CrI3 layer with intrinsic magnetism leads to efficient control and tunability of the observed spin-correlated anomalous Nernst effect. Moreover, a large dimensionless figure of merit of ∼6 and a power factor of ∼3.8×1011/τ∘ Wm−1K−2s−1 near the Fermi level at 300 K endorse the rejuvenated TE effect. The strong relativistic spin–orbit coupling validates the significant correlation of TE properties with intrinsic magnetic configuration. The present study underscores the significance of the magnetic proximity-governed TE effect in vdW heterostructures to engineer low-dimensional TE devices.

The vertical integration of distinct 2D materials in van der Waals (vdW) heterostructures provides the opportunity for interface engineering and modulation of electronic as well as optical properties. However, scarce experimental studies reveal many challenges for vdW heterostructures, hampering the fine-tuning of their electronic and optical functionalities. Optically active MXenes, the most recent member of the 2D family, with excellent hydrophilicity, rich surface chemistry, and intriguing optical properties, are a novel 2D platform for optoelectronics applications. Coupling MXenes with various 2D materials into vdW heterostructures can open new avenues for the exploration of physical phenomena of novel quantum-confined nanostructures and devices. Therefore, the fundamental basis and recent findings in vertical vdW heterostructures composed of MXenes as a primary component and other 2D materials as secondary components are examined. Their robust designs and synthesis approaches that can push the boundaries of light-harvesting, transition, and utilization are discussed, since MXenes provide a unique playground for pursuing an extraordinary optical response or unusual light conversion features/functionalities. The recent findings are finally summarized, and a perspective for the future development of next-generation vdW multifunctional materials enriched by MXenes is provided.