Two-dimensional Van Der Waals Heterostructures Research Articles

A synapse with low power consumption based on MoTe2/SnS2heterostructure.

The use of two-dimensional materials and van der Waals heterostructures holds great potential for improving the performance of memristors Here, we present SnS2/MoTe2heterostructure synaptic transistors. Benefiting from the ultra-low dark current of the heterojunction, the power consumption of the synapse is only 19 pJ per switching under 0.1 V bias, comparable to that of biological synapses. The synaptic device based on the SnS2/MoTe2demonstrates various synaptic functionalities, including short-term plasticity, long-term plasticity, and paired-pulse facilitation (PPF). In particular, the synaptic weight of the excitatory postsynaptic current (EPSC) can reach 109.8%. In addition, the controllability of the long-term potentiation (LTP) and long-term depression (LTD) are discussed. The dynamic range (Gmax/Gmin) and the symmetricity values of the synaptic devices are approximately 16.22 and 6.37, and the non-linearity is 1.79. Our study provides the possibility for the application of 2D material synaptic devices in the field of low-power information storage.
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Double Superconducting Dome of Quasi Two-Dimensional TaS2 in Non-Centrosymmetric van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (2D-vdWHs) based on transition metal dichalcogenides (TMDs) provide unparalleled control over electronic properties. However, the interlayer coupling is challenged by the interfacial misalignment and defects, which hinders a comprehensive understanding of the intertwined electronic orders, especially superconductivity and charge density wave (CDW). Here, by using pressure to regulate the interlayer coupling of non-centrosymmetric 6R-TaS2 vdWHs, we observe an unprecedented phase diagram in TMDs. This phase diagram encompasses successive suppression of the original CDW states from alternating H-layer and T-layer configurations, the emergence and disappearance of a new CDW-like state, and a double superconducting dome induced by different interlayer coupling effects. These results not only illuminate the crucial role of interlayer coupling in shaping the complex phase diagram of TMD systems but also pave a new avenue for the creation of a novel family of bulk heterostructures with customized 2D properties.

Moiré band renormalization due to lattice mismatch in bilayer graphene

We investigated the band renormalization caused by the compressive-strain-induced lattice mismatch in parallel AA stacked bilayer graphene using two complementary methods: the tight-binding approach and the low-energy continuum theory. While a large mismatch does not alter the low-energy bands, a small one reduces the bandwidth of the low-energy bands along with a decrease in the Fermi velocity. In the tiny-mismatch regime, the low-energy continuum theory reveals that the long-period moiré pattern extensively renormalizes the low-energy bands, resulting in a significant reduction of bandwidth. Meanwhile, the Fermi velocity exhibits an oscillatory behavior and approaches zero at specific mismatches. However, the resulting low-energy bands are not perfectly isolated flat, as seen in twisted bilayer graphene at magic angles. These findings provide a deeper understanding of moiré physics and offer valuable guidance for related experimental studies in creating moiré superlattices using two-dimensional van der Waals heterostructures.

First-principles study of perpendicular magnetic anisotropy of CrI3/In2Se3 Van der Waals heterostructure

Two-dimensional multiferroic van der Waals (vdW) heterostructures are promising for nanoscale and multifunctional spintronic devices. Controllable perpendicular magnetic anisotropy in vdW heterostructure often plays a central role in stabilizing ferromagnetic order and lowering energy consumption. Here, we carry out first-principles calculations for CrI3/In2Se3 vdW heterostructure and investigate how its magnetic anisotropy is interrelated to the electronic structure. We show that the semiconducting nature of CrI3 is preserved but band gap decreases in monolayer-CrI3/In2Se3 and bilayer-CrI3/In2Se3 vdW heterostructures. The magnetoelectric effect of CrI3/In2Se3 vdW heterostructure enhances the magnetic moment of CrI3, but decreases the perpendicular magnetic anisotropy of CrI3. I-p orbital coupling matrix element is noteworthy in regard to the modulation of magnetic anisotropy. The MAE contribution of Cr atom slightly enhances but is still negligible by the reason of countervailing contribution of d-orbital coupling matrix elements. By reversing the ferroelectric polarization of In2Se3, the easy magnetization axis of monolayer CrI3 can be switched. These results will help to understand the magnetic anisotropy of multiferroic vdW heterostructure and may enable the development of nonvolatile memory devices.

Electric-field-controlled Schottky barriers in BSe/M2CF2 (M = Ta, W) van der Waals heterostructures: A computational study

Constructing two-dimensional (2D) van der Waals (vdW) heterostructures is regarded as an effective strategy to explore novel properties and enhance the performance of 2D materials. Herein, taking full advantage of boron selenide (BSe) and MXenes, 2D BSe/MXene vdW heterostructures are designed and their electronic and interfacial properties are investigated via first-principles calculations, by using BSe/Ta2CF2 and BSe/W2CF2 as models. The band structures of the BSe and M2CF2 (M = Ta, W) layers in the BSe/M2CF2 heterostructures are preserved quite well due to the weak vdW interlayer interaction. The Fermi level shifts towards the conduction band minimum of the BSe layer, leading to an n-type Schottky contact. Interestingly, the interface charge redistribution in BSe/W2CF2 is more pronounced compared to BSe/Ta2CF2, which can be attributed to the disparity in electronegativity between Ta and W. In particular, the Schottky barrier heights of both heterostructures vary linearly with the vertical electric field within a certain range. This allows for the tuning of the Schottky contact from n-type to p-type. However, the variation rate in BSe/W2CF2 is comparatively lower than that in BSe/Ta2CF2. Moreover, when the positive electric field exceeds 1.0 V/Å, a transition from the Schottky contact to the Ohmic contact in BSe/W2CF2 occurs, whereas the contact remains the Schottky contact in BSe/W2CF2. This work offers valuable insights into developing electronic nano-devices with tunable Schottky barriers using vdW heterostructures.

Engineering MPC-Assisted Heterojunctional Photo-Oxidation Tailored by Interfacial Design of a P-Modulated C3N4 Heterojunction for Improved Aerobic Alcohol Oxidation.

The creation of two-dimensional van der Waals (VDW) heterostructures is a sophisticated approach to enhancing photocatalytic efficiency. However, challenges in electron transfer at the interfaces often arise in these heterostructures due to the varied structures and energy barriers of the components involved. This study presents a novel method for constructing a VDW heterostructure by inserting a phosphate group between copper phthalocyanine (CuPc) and boron-doped, nitrogen-deficient graphitic carbon nitride (BCN), referred to as Cu/PO4-BCN. This phosphate group serves as a charge mediator, enabling effective charge transfer within the heterostructure, thus facilitating electron flow from BCN to CuPc upon activation. As a result, the photogenerated electrons are effectively utilized by the catalytic Cu2+ core in CuPc, achieving a conversion efficiency of 96% for benzyl alcohol (BA) and a selectivity of 98.8% for benzyl aldehyde (BAD) in the presence of oxygen as the sole oxidant and under illumination. Notably, the production rate of BAD is almost 8 times higher than that observed with BCN alone and remains stable over five cycles. The introduction of interfacial mediators to enhance electron transfer represents a pioneering and efficient strategy in the design of photocatalysts, enabling the proficient transformation of BA into valuable derivatives.

Photothermionic Effect-Assisted Ultrafast Charge Transfer in NbS2/MoS2 Heterostructure.

Two-dimensional (2D) van der Waals heterostructures (vdW HSs) composed of transition metal dichalcogenides (TMDCs) have emerged as frontrunners in the optoelectronics field, owing to their exceptional optical and electrical properties. Recent research on the intrinsic interlayer charge transfer mechanism has been primarily focused on the Type II HSs, while metal-semiconductor (MS) vertical HSs, promising for advancing photodetector technology, have received comparatively less attention. Here, we reveal the first experimental observation of photothermionic effect-assisted ultrafast interlayer charge transfer in the NbS2/MoS2 heterostructure using femtosecond transient absorption technology and first-principles calculations, effectively ignoring the Schottky barrier height. We demonstrate that within 500 fs, charge transfer occurs from NbS2 to MoS2 in the heterostructure, resulting in supplementary carrier generation in the visible spectrum when excited with infrared light below the MoS2 bandgap, at wavelengths of 1030 and 1500 nm. Such promising characteristics of 2D NbS2-semiconductor heterostructures offer a potential platform for synergistically combining low contact resistance with broadband photocarrier generation, marking a significant advancement in optoelectronics and light harvesting.

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.

Ideal Photodetector Based on WS2/CuInP2S6 Heterostructure by Combining Band Engineering and Ferroelectric Modulation.

Two-dimensional van der Waals (2D vdW) heterostructure photodetectors have garnered significant attention for their potential applications in next-generation optoelectronic systems. However, current 2D vdW photodetectors inevitably encounter compromises between responsivity, detectivity, and response time due to the absence of multilevel regulation for free and photoexcited carriers, thereby restricting their widespread applications. To address this challenge, we propose an efficient 2D WS2/CuInP2S6 vdW heterostructure photodetector by combining band engineering and ferroelectric modulation. In this device, the asymmetric conduction and valence band offsets effectively block the majority carriers (free electrons), while photoexcited holes are efficiently tunneled and rapidly collected by the bottom electrode. Additionally, the ferroelectric CuInP2S6 layer generates polarization states that reconfigure the built-in electric field, reducing dark current and facilitating the separation of photocarriers. Moreover, photoelectrons are trapped during long-distance lateral transport, resulting in a high photoconductivity gain. Consequently, the device achieves an impressive responsivity of 88 A W-1, an outstanding specific detectivity of 3.4 × 1013 Jones, and a fast response time of 37.6/371.3 μs. Moreover, the capability of high-resolution imaging under various wavelengths and fast optical communication has been successfully demonstrated using this device, highlighting its promising application prospects in future optoelectronic systems.

Harnessing Van der Waals CrPS4 and Surface Oxides for Nonmonotonic Preset Field Induced Exchange Bias in Fe3GeTe2.

Two-dimensional van der Waals (vdW) heterostructures are an attractive platform for studying exchange bias due to their defect-free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnetic material, possesses uncompensated magnetic spins in a single layer, rendering it a promising candidate for exploring exchange bias phenomena. Recent findings have highlighted that naturally oxidized vdW ferromagnetic Fe3GeTe2 exhibits exchange bias, attributed to the antiferromagnetic coupling of its ultrathin surface oxide layer (O-FGT) with the underlying unoxidized Fe3GeTe2. Anomalous Hall measurements are employed to scrutinize the exchange bias within the CrPS4/(O-FGT)/Fe3GeTe2 heterostructure. This analysis takes into account the contributions from both the perfectly uncompensated interfacial CrPS4 layer and the interfacial oxide layer. Intriguingly, a distinct and nonmonotonic exchange bias trend is observed as a function of temperature below 140 K. The occurrence of exchange bias induced by a "preset field" implies that the prevailing phase in the polycrystalline surface oxide is ferrimagnetic Fe3O4. Moreover, the exchange bias induced by the ferrimagnetic Fe3O4 is significantly modulated by the presence of the van der Waals antiferromagnetic CrPS4 layer, forming a heterostructure, along with additional iron oxide phases within the oxide layer. These findings underscore the intricate and complex nature of exchange bias in van der Waals heterostructures, highlighting their potential for tailored manipulation and control.

A Cu3BHT-Graphene van der Waals Heterostructure with Strong Interlayer Coupling for Highly Efficient Photoinduced Charge Separation.

Two-dimensional van der Waals heterostructures (2D vdWhs) are of significant interest due to their intriguing physical properties critically defined by the constituent monolayers and their interlayer coupling. Synthetic access to 2D vdWhs based on chemically tunable monolayer organic 2D materials remains challenging. Herein, the fabrication of a novel organic-inorganic bilayer vdWh by combining π-conjugated 2D coordination polymer (2DCP, i.e., Cu3BHT, BHT = benzenehexathiol) with graphene is reported. Monolayer Cu3BHT with detectable µm2-scale uniformity and atomic flatness is synthesized using on-water surface chemistry. A combination of diffraction and imaging techniques enables the determination of the crystal structure of monolayer Cu3BHT with atomic precision. Leveraging the strong interlayer coupling, Cu3BHT-graphene vdWh exhibits highly efficient photoinduced interlayer charge separation with a net electron transfer efficiency of up to 34% from Cu3BHT to graphene, superior to those of reported bilayer 2D vdWhs and molecular-graphene vdWhs. This study unveils the potential for developing novel 2DCP-based vdWhs with intriguing physical properties.

Field-free spin–orbit torque switching of magnetic tunnel junction structure based on two-dimensional van der Waals WTe2

In recent years, two-dimensional van der Waals (2D vdWs) heterostructures have attracted great research interest due to their great potential in fundamental physics research and spintronic devices (such as MTJs). Due to its excellent scalability, controllable magnetism and out-of-plane anisotropy, the compact nonvolatile memory controller (NV-MC) based on spintronics is expected to solve the memory bottle-neck problem. At present, a series of in-depth studies have been conducted on advanced 2D vdWs materials, such as MoS2, WSe2, and Fe3GeTe2 (FGT). The results show that the 2D vdWs materials have great TMR value and high SOT switching efficiency, both theoretically reported and experimentally verified. In the paper, a novel MTJ device based on the FGT/WTe2 heterostructure is proposed. In the absence of an external magnetic field, the magnetization direction of the MTJ free layer can still be reversed with certainty when the unipolar write current reaches about 5 mA. Moreover, the DMI effect generated between 2-D material/FM interfaces is also considered, which can promote the performance of SOT-MTJ without the external field. The reading reliability of SOT-MRAM is improved in comparison with the traditional CoFeB-based MTJ device.

Fast and robust multilevel optoelectronic memory based on van der Waals heterostructure

Optoelectronic memory (OEM) has attracted tremendous attention for its great potential to boost the storage capacity of memory chips and break through the von Neumann bottleneck in the post-Moore era. Two-dimensional (2D) van der Waals (vdW) heterostructures, formed by artificially stacking different 2D layered materials, offer tremendous possibilities in OEMs due to their extraordinary capability to integrate and process optical/electrical signals. However, the realization of 2D vdW OEMs with high writing speed and robust memory performance has long been challenging. Here, we report a 2D vdW OEM consisting of tungsten diselenide (WSe2) and hexagonal boron nitride, which functions based on the fast charge transfer dynamics at a 2D interface. The OEM demonstrates high writing speed reaching up to 50 μs, approximately one order of magnitude faster than those of other 2D OEMs. Moreover, the outstanding robustness of such OEM is demonstrated by long retention time exceeding 14 days, together with a broad temperature endurance window from 100 to 420 K. Additionally, through continuously switching laser pulse on the OEM, we achieve 17 distinct current levels (over 4-bit storage) with random access. Our findings envision 2D vdW heterostructure-based OEM as a potential platform to overcome the “memory wall” in the conventional von Neumann configuration and to promote a promising paradigm for big data storage.

Sliding-mediated ferroelectric phase transition in CuInP2S6 under pressure

Interlayer stacking order has recently emerged as a unique degree of freedom to control crystal symmetry and physical properties in two-dimensional van der Waals (vdW) materials and heterostructures. By tuning the layer stacking pattern, symmetry-breaking and electric polarization can be created in otherwise non-polar crystals, whose polarization reversal depends on the interlayer sliding motion. Herein, we demonstrate that in a vdW layered ferroelectric, its existing polarization is closely coupled to the interlayer sliding driven by hydrostatic pressure. Through combined structural, electrical, vibrational characterizations, and theoretical calculations, we clearly map out the structural evolution of CuInP2S6 under pressure. A tendency toward a high polarization state is observed in the low-pressure region, followed by an interlayer-sliding-mediated phase transition from a monoclinic to a trigonal phase. Along the transformation pathway, the displacive-instable Cu ion serves as a pivot point that regulates the interlayer interaction in response to external pressure. The rich phase diagram of CuInP2S6, which is enabled by stacking orders, sheds light on the physics of vdW ferroelectricity and opens an alternative route to tailoring long-range order in vdW layered crystals.

Two-dimensional Van der Waals heterostructures based chalcogenide for photovoltaic applications: a DFT study

Two-dimensional Van der Waals heterostructures based chalcogenide for photovoltaic applications: a DFT study

Tailored Synthesis of Heterogenous 2D TMDs and Their Spectroscopic Characterization

Two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) show great potential across various applications. However, synthesizing large-scale structures poses challenges owing to the intricate growth parameters, forming unexpected hybrid film structures. Thus, precision in synthesis and thorough structural analysis are essential aspects. In this study, we successfully synthesized large-scale structured 2D transition metal dichalcogenides (TMDs) via chemical vapor deposition using metal oxide (WO3 and MoO3) thin films and a diluted H2S precursor, individual MoS2, WS2 films and various MoS2/WS2 hybrid films (Type I: MoxW1−xS2 alloy; Type II: MoS2/WS2 vdWH; Type III: MoS2 dots/WS2). Structural analyses, including optical microscopy, Raman spectroscopy, transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy, and cross-sectional imaging revealed that the A1g and E2g modes of WS2 and MoS2 were sensitive to structural variations, enabling hybrid structure differentiation. Type II showed minimal changes in the MoS2′s A1g mode, while Types I and III exhibited a ~2.8 cm−1 blue shift. Furthermore, the A1g mode of WS2 in Type I displayed a 1.4 cm−1 red shift. These variations agreed with the TEM-observed microstructural features, demonstrating strain effects on the MoS2–WS2 interfaces. Our study provides insights into the structural features of diverse hybrid TMD materials, facilitating their differentiation through Raman spectroscopy.

Field-free switching of perpendicular magnetization by two-dimensional PtTe2/WTe2 van der Waals heterostructures with high spin Hall conductivity.

The key challenge of spin-orbit torque applications lies in exploring an excellent spin source capable of generating out-of-plane spins while exhibiting high spin Hall conductivity. Here we combine PtTe2 for high spin conductivity and WTe2 for low crystal symmetry to satisfy the above requirements. The PtTe2/WTe2 bilayers exhibit a high in-plane spin Hall conductivity σs,y ≈ 2.32 × 105 × ħ/2e Ω-1 m-1 and out-of-plane spin Hall conductivity σs,z ≈ 0.25 × 105 × ħ/2e Ω-1 m-1, where ħ is the reduced Planck's constant and e is the value of the elementary charge. The out-of-plane spins in PtTe2/WTe2 bilayers enable the deterministic switching of perpendicular magnetization at room temperature without magnetic fields, and the power consumption is 67 times smaller than that of the Pt control case. The high out-of-plane spin Hall conductivity is attributed to the conversion from in-plane spin to out-of-plane spin, induced by the crystal asymmetry of WTe2. Our work establishes a low-power perpendicular magnetization manipulation based on wafer-scale two-dimensional van der Waals heterostructures.

A 2D-0D-2D Sandwich Heterostructure toward High-Performance Room-Temperature Gas Sensing.

The construction of two-dimensional (2D) van der Waals (vdW) heterostructures over black phosphorus (BP) has been attracting significant attention to better utilize its inherent properties. The sandwich of zero-dimensional (0D) noble metals within BP-based vdW heterostructures can provide efficient catalytic channels, modulating their surface redox potentials and therefore inducing versatile functionalities. Herein, we realize a 2D WS2-Au-BP heterostructure, in which Au nanoparticles are connected between BP and WS2 via ionic bonds. The ultralow conduction band minimum position, the reduced adsorption energies of O2, and the increased dissociation barrier energy of O2- into 2O contribute greatly to improving the long-term stability of BP in the air. The formation of heterostructures can reduce the potential barrier energy in target gas molecules, thus enhancing the absorption energy and charge transfer. Taking the paramagnetic NO2 gas molecules as a representative, a stable response magnitude of 2.11 to 100 ppb NO2 is achieved for 80 days, which is far larger than the initial responses of most BP-based materials. A practical gas sensing system is also developed to demonstrate its real-world implementation. This work provides a promising demonstration of 0D noble metal within 2D BP-based vdW heterostructure for simultaneously improving the long-term stability and room-temperature reversible gas sensing.

2D van der Waals Heterostructure with Tellurene Floating-Gate for Wide Range and Multi-Bit Optoelectronic Memory.

Intensive research on optoelectronic memory (OEM) devices based on two-dimensional (2D) van der Waals heterostructures (vdWhs) is being conducted due to their distinctive advantages for electrical-optical writing and multilevel storage. These features make OEM a promising candidate for the logic of reconfigurable operations. However, the realization of nonvolatile OEM with broadband absorption (from visible to infrared) and a high switching ratio remains challenging. Herein, we report a nonvolatile OEM based on a heterostructure consisting of rhenium disulfide (ReS2), hexagonal boron nitride (hBN) and tellurene (2D Te). The 2D Te-based floating-gate (FG) device exhibits excellent performance metrics, including a high switching on/off ratio (∼106), significant endurance (>1000 cycles) and impressive retention (>104 s). In addition, the narrow band gap of 2D Te endows the device with broadband optical programmability from the visible to near-infrared regions at room temperature. Moreover, by applying different gate voltages, light wavelengths, and laser powers, multiple bits can be successfully generated. Additionally, the device is specifically designed to enable reconfigurable inverter logic circuits (including AND and OR gates) through controlled electrical and optical inputs. These significant findings demonstrate that the 2D vdWhs with a 2D Te FG are a valuable approach in the development of high-performance OEM devices.

Enhanced photocatalytic water splitting with two-dimensional van der Waals heterostructures of BAs/WTeSe

DFT predicts stable boron arsenide-Janus monolayer heterostructures with tunable bandgaps, high light absorption, and good carrier mobility, making them ideal for photocatalytic water splitting.