Van Der Waals Heterostructures Research Articles

Two-dimensional ferroelastic semiconductors InXY (X = S, Se; Y = Cl, Br, I): Promising candidates for photocatalytic water splitting with tunable electronic anisotropy

We have identified a class of two-dimensional ferroelastic monolayers, denoted as InXY (where X = S, Se; Y = Cl, Br, I), through first-principles calculations. The dynamic, thermal, and mechanical stabilities of these InXY monolayers are validated by phonon dispersion spectra, AIMD calculations, and elastic constants, respectively. These monolayers exhibit semiconducting behavior with bandgaps ranging from 1.94 to 2.85 eV and possess excellent ferroelasticity with strong ferroelastic signals and moderate ferroelastic switching barriers. Notably, the band edge positions of InSBr and InSI monolayers are observed to stride the water redox potentials at pH = 0, indicating their potential as photocatalysts for water splitting in acidic environments. We also explored the effects of biaxial strain on the band edge alignments and photocatalytic performance of these monolayers. Moreover, the InXY monolayers exhibit excellent anisotropic optical absorption across the visible to ultraviolet regions, along with high anisotropic carrier transport. The coupling of ferroelastic and anisotropic properties in these monolayers offers promising opportunities for designing controllable electronic devices, thereby expanding their potential applications in multifunctional materials. Our findings reveal that the InXY monolayers are promising candidates for efficient photocatalytic water splitting and controllable optoelectronic applications.

Interfacial negative biexcitons in a monolayer WS2/InGaN quantum dots heterostructure

In this work, we fabricated a Van der Waals heterostructure of monolayer (ML) WS2 and InGaN quantum dots (QDs). This heterostructure is divided into coupled and uncoupled regions based on the thickness of the inserted hBN layer. Upon measuring its PL spectra, we identified an interfacial negative biexciton, which consists of a trion in ML WS2 and an exciton in QDs, in the coupled region. This interfacial negative biexciton features the negative charge of the trion and the quantum confinement of QDs, with its relative intensity showing a strong dependence on the excitation photon energy and featuring a significant threshold. Our work highlights the effective coupling within the mixed-dimensional heterostructure, offering new prospects for the study of many-body physics.

All-2D asymmetric self-powered photodetectors with ultra-fast photoresponse based on Gr/WSe2/NbSe2 van der Waals heterostructure

The rise of smart wearable devices has driven the demand for flexible, high-performance optoelectronic devices with low power and easy high-density integration. Emerging Two-dimensional (2D) materials offer promising solutions. However, the use of 3D metal in traditional 2D devices often leads to Fermi-level pinning, compromising device performance. 2D metallic materials, such as graphene and 2H-phase NbSe2, present a new avenue for addressing this issue and constructing high-performance, low-power photodetectors. In this work, we designed an all-2D asymmetric contacts photodetector using Gr and NbSe2 as electrodes for the 2D semiconductor WSe2. The asymmetric Schottky barriers and built-in electric fields facilitated by this architecture resulted in outstanding photovoltaic characteristics and self-powered photodetection. Under zero bias, the device exhibited a responsivity of 287 mA/W, a specific detectivity of 5.3 × 1011 Jones, and an external quantum efficiency of 88%. It also demonstrated an ultra-high light on/off ratio (1.8 × 105), ultra-fast photoresponse speeds (80/72 μs), broad-spectrum responsiveness (405–980 nm), and exceptional cycling stability. The applications of the Gr/WSe2/NbSe2 heterojunction in imaging and infrared optical communication have been explored, underscoring its significant potential. This work offers an idea to construct all-2D ultrathin optoelectronic devices.

Prediction of material stability of two-dimensional semiconductors: An interpretable machine learning perspective

Despite significant advancements in leveraging artificial intelligence (AI) for drug design, materials science, and other fields, the question of how each dataset feature influences a target metric—essential for constructing better predictive models and targeted materials design—remains largely unaddressed. In this study, we explored the application of interpretable machine learning (ML) techniques to the inverse design of two-dimensional (2D) semiconductor materials, a critical yet underexplored area within the AI4Science domain. Our approach utilized a dataset from the C2DB database, incorporating advanced feature engineering and data imputation strategies to predict material stability, a key determinant of a materials industrial and academic value. Through the calculation of Shapley additive explanation scores and counterfactual analysis, we provided a nuanced understanding of feature contributions toward material stability, enabling the targeted design of 2D semiconductors with optimized properties. This work not only fills the gap in the current literature by emphasizing the role of interpretability in materials design but also demonstrates the potential of interpretable ML in guiding the development of novel materials with enhanced performance characteristics.

Facile formation of van der Waals metal contact with III-nitride semiconductors

Metal–semiconductor contacts play a pivotal role in controlling carrier transport in the fabrication of modern electronic devices. The exploration of van der Waals (vdW) metal contacts in semiconductor devices can potentially mitigate Fermi-level pinning at the metal–semiconductor interface, with particular success in two-dimensional layered semiconductors, triggering unprecedented electrical and optical characteristics. In this work, for the first time, we report the direct integration of vdW metal contacts with bulk wide bandgap gallium nitride (GaN) by employing a dry transfer technique. High-angle annular dark-field scanning transmission electron microscopy explicitly illustrates the existence of a vdW gap between the metal electrode and GaN. Strikingly, compared with devices fabricated with electron beam-evaporated metal contacts, the vdW contact device exhibits a responsivity two orders of magnitude higher with a significantly suppressed dark current in the nanoampere range. Furthermore, by leveraging the high responsivity and persistent photoconductivity obtained from vdW contact devices, we demonstrate imaging, wireless optical communication, and neuromorphic computing functionality. The integration of vdW contacts with bulk semiconductors offers a promising architecture to overcome device fabrication challenges, forming nearly ideal metal–semiconductor contacts for future integrated electronics and optoelectronics.

Two-dimensional Cr2Cl3S3 Janus magnetic semiconductor with large magnetic exchange interaction and high-TC

Abstract The two-dimensional (2D) Janus monolayers are promising in spintronic device application due to their enhanced magnetic couplings and Curie temperatures. Van der Waals CrCl3 monolayer has been experimentally proved to have an in-plane magnetic easy axis and a low Curie temperature of 17 K, which will limit its application in spintronic devices. In this work, we propose a new Janus monolayer Cr2Cl3S3 based on the first principles calculations. The phonon dispersion and elastic constants confirm that Janus monolayer Cr2Cl3S3 is dynamically and mechanically stable. Our Monte Carlo simulation results based on magnetic exchange constants reveal that Janus monolayer Cr2Cl3S3 is an intrinsic ferromagnetic semiconductor with T C of 180 K, which is much higher than that of CrCl3 due to the enhanced ferromagnetic coupling caused by S substitution. Moreover, the magnetic easy axis of Janus Cr2Cl3S3 can be tuned to the perpendicular direction with a large magnetic anisotropy energy (MAE) of 142 μeV/Cr. Furthermore, the effect of biaxial strain on the magnetic property of Janus monolayer Cr2Cl3S3 is evaluated. It is found that the Curie temperature is more robust under tensile strain. This work indicates that the Janus monolayer Cr2Cl3S3 presents increased Curie temperature and out-of-plane magnetic easy axis, suggesting greater application potential in 2D spintronic devices.

Composition Modulation-Mediated Band Alignment Engineering from Type I to Type III in 2D vdW Heterostructures.

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe2/Bi2Te3- xSex vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe2/Bi2Te3) to Type III (WSe2/Bi2Se3). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 AW-1 and detectivity of 2.91×1012 Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

Nematic Ising superconductivity with hidden magnetism in few-layer 6R-TaS2

In van der Waals heterostructures (vdWHs), the manipulation of interlayer stacking/coupling allows for the construction of customizable quantum systems exhibiting exotic physics. An illustrative example is the diverse range of states of matter achieved through varying the proximity coupling between two-dimensional (2D) quantum spin liquid (QSL) and superconductors within the TaS2 family. This study presents a demonstration of the intertwined physics of spontaneous rotational symmetry breaking, hidden magnetism, and Ising superconductivity (SC) in the three-fold rotationally symmetric, non-magnetic natural vdWHs 6R-TaS2. A distinctive phase emerges in 6R-TaS2 below a characteristic temperature (T*) of approximately 30 K, which is characterized by a remarkable set of features, including a giant extrinsic anomalous Hall effect (AHE), Kondo screening, magnetic field-tunable thermal hysteresis, and nematic magneto-resistance. At lower temperatures, a coexistence of nematicity and Kondo screening with Ising superconductivity is observed, providing compelling evidence of hidden magnetism within a superconductor. This research not only sheds light on unexpected emergent physics resulting from the coupling of itinerant electrons and localized/correlated electrons in natural vdWHs but also emphasizes the potential for tailoring exotic quantum states through the manipulation of interlayer interactions.

Theoretical design of graphene/tungsten dichalcogenide (Gr/WX2, X=S, Se) heterostructures used for anode materials of LIBs from DFT calculations

2D van der Waals heterostructures are highly promising for Lithium-ion batteries (LIBs), offering excellent energy capacity and extended lifespan when used as anodes. In this study, we systematically investigate the geometrical structures and electronic properties of graphene/tungsten dichalcogenide (Gr/WX2, X=S, Se) heterostructures, focusing on lithium (Li) atom adsorption and movement in these materials using first-principles calculations through density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Our results reveal that the Gr/WX2 heterostructures possess excellent structural stability, with mechanical strength validated by their elastic constants. The diffusion barriers of Li atoms in Gr/WS2 and Gr/WSe2 heterostructures are competitive compared with other anode materials. Additionally, the Li diffusion coefficients at 300 K for Gr/WS2 and Gr/WSe2 are also surpassing those observed in pristine Gr. Furthermore, Gr effectively lowers the average open-circuit voltage (OCV) of Gr/WX2 heterostructures to optimal levels, with Gr/WS2 at 0.29V and Gr/WSe2 at 0.25V. The Gr/WS2 and Gr/WSe2 heterostructures after adsorption of Li exhibit remarkable thermal stability at 300K, maintaining their structural integrity throughout the simulation period. Our research provides comprehensive theoretical guidance for the practical development and application of Gr/WX2 heterostructures as anode materials in LIBs.

Investigation on the carrier dynamics and energy band of the CVD-grown large area WS2/Bi2O2Se heterostructure

Bismuth oxyselenide (Bi2O2Se) emerged as a prominent member of the quasi-2D layered material family, possesses appealing characteristics for optoelectronic applications including impressive environmental stability and high carrier mobility. Recently, although significant advancements have been made in the initial research on the optoelectronic characteristics of Bi2O2Se-based heterostructures, there remains a lack of comprehensive study on the carrier dynamics and energy band within these structures. In this work, large-area (1 cm × 1 cm) continuous Bi2O2Se and monolayer WS2 films were grown by chemical vapor deposition (CVD) method, and the related WS2/Bi2O2Se heterostructures were successfully constructed. Triple decay processes with lifetimes ofτ1 ∼ 298 ps, τ2∼37 ps andτ3 ∼ 1.58 ns are observed through time-resolved photoluminesce (TRPL), which were attributed to the recombination of neutral excitons, trions, and interlayer excitons, respectively. Then, the energy band structure was investigated through x-ray photoelectron spectroscopy, revealing a type-Ⅱ band alignment at the heterointerface. The valence band offset was 0.19 eV, while conduction band offset was approximately 1.09 eV as confirmed by ultraviolet photoelectron spectroscopy. As a result, the WS2/Bi2O2Se junction enhances charge transfer and interlayer interactions by the aid of interface build-in electric field. These results showcase the potential of integrating Bi2O2Se with other 2D semiconductors to form heterostructures possessing novel charge dynamic behaviors, and provide valuable understanding into the functionality of optoelectronic devices that rely on these 2D heterostructures.

Schottky-barrier-free contacts with Janus WSSe 2D semiconductor using surface-engineered MXenes

Forming low-barrier contacts at metal/semiconductor interfaces is an important research area. The Fermi level pinning (FLP) effect is induced by strong interactions between 3D metal/semiconductor interfaces. By using a 2D metal in contact with a 2D semiconductor, the FLP effect can be weakened considerably, resulting in a reduction of the Schottky barrier height (SBH). Surface-engineered MXenes are the best electrode materials available presently. Therefore, herein, the interfacial properties of surface-engineered MXenes (M2CT2) (M=Hf, Mo, Ta, Zr, Nb; T = F, OH, O)/Janus WSSe (WSSe) heterostructures are systematically investigated using first-principles calculations. OH-terminated MXenes [M2C(OH)2]/WSSe heterostructures are found to form n-type ohmic contacts, and the tunnelling probabilities are high. O-terminated MXenes (M2CO2)/WSSe heterostructures tend to form p-type ohmic contacts and p-type Schottky contacts with a very low SBH, and the tunnelling probability is low. Conversely, F-terminated MXenes (M2CF2)/WSSe heterostructures form n-type and p-type Schottky contacts readily and n-type ohmic contacts (Hf2CF2/WSSe and Zr2CF2/SWSe heterostructures) partially, and the tunnelling probability is found to be low. In addition, the synergistic action of interface and intrinsic dipoles can effectively tune the contact type and SBH of the M2CT2/WSSe heterostructures, transforming the Mo2CF2/WSSe and Nb2CF2/WSSe heterostructures from n-type to p-type Schottky contacts. Through this novel strategy, an SB-free contact can be readily obtained, especially for OH-terminated MXenes (which are the most ideal electrode materials), followed by O-terminated MXenes. This work provides an important reference for the selection of suitable electrode materials and a novel approach for the development of high-performance electronic devices based on Janus WSSe.

Strain fingerprinting of exciton valley character in 2D semiconductors

Intervalley excitons with electron and hole wavefunctions residing in different valleys determine the long-range transport and dynamics observed in many semiconductors. However, these excitons with vanishing oscillator strength do not directly couple to light and, hence, remain largely unstudied. Here, we develop a simple nanomechanical technique to control the energy hierarchy of valleys via their contrasting response to mechanical strain. We use our technique to discover previously inaccessible intervalley excitons associated with K, Γ, or Q valleys in prototypical 2D semiconductors WSe2 and WS2. We also demonstrate a new brightening mechanism, rendering an otherwise “dark” intervalley exciton visible via strain-controlled hybridization with an intravalley exciton. Moreover, we classify various localized excitons from their distinct strain response and achieve large tuning of their energy. Overall, our valley engineering approach establishes a new way to identify intervalley excitons and control their interactions in a diverse class of 2D systems.

Study on the interface electronic structure, strain modulation and transport properties of composite heterojunction of 2D semimetal TiS2 and MX2 (M=Mo, W, Cr, Zr, Hf; X=S, Se, Te) semiconductor

Ohmic contacts play a crucial role in realizing high-performance electronic devices based on two-dimensional materials. The contact between semimetals and semiconductors can mitigate the formation of metal-induced gap states (MIGS), thereby reducing the SBH, enhancing the efficiency of high charge injection, and facilitating the establishment of ohmic contacts. This study involves a systematic exploration of the contact characteristics between the two-dimensional semimetal TiS2 and semiconductor MX2 (M = Mo, W, Cr, Zr, Hf; X = S, Se, Te) through first-principles calculations. It is found that the TiS2/MoSe2 and TiS2/WSe2 heterojunction achieve ohmic contact. Investigations into their transport properties reveal that significant currents can be observed at relatively low voltages, indicating excellent transport performance of these heterojunctions. The TiS2/CrSe2 and TiS2/HfSe2 contact heterojunctions also show low Schottky barrier height (SBH), with the barrier height being adjustable under strain. The SBH of TiS2/CrSe2 and TiS2/HfSe2 heterojunctions are very close to zero under stresses of 4 % and −4%, respectively. This also implies that our research can offer valuable guidance for the development of adjustable Schottky nano-devices and high-performance optoelectronic devices.

Reconfigurable dielectric engineered WSe2/HZO mem-transistor

Abstract Hybrid systems coupling 2D semiconductors with functional ferroelectrics are attracting increasing attention due to their excellent electronic/optoelectronic properties and new functionalities through the multiple heterointerface interaction. In our device architecture, interfacial states are introduced on the ferroelectric Hf0.5Zr0.5O2 thin film as gate dielectric layer for charge trapping effect. Utilizing the collaborative effects of charge trapping and ferroelectric polarization behavior, a multifunctional 2D WSe2/HZO memtransistor is demonstrated with ultra-low off-state (dark) current of 10-13 A, high on/off ratio of 106 and linear conductance update. This device exhibits reliable memory properties and tunable synaptic functions including short-term plasticity (STP)/ long-term plasticity (LTP), paired pulse facilitation (PPF), spike-timing dependent plasticity (STDP), synaptic potentiation/depression, and filtering in a single device. Extensive endurance tests ensure robust stability (1000 switching cycles, 2000 s holding time) and the synaptic weight update in the device exhibits excellent linearity. Based on the experimental data, our devices eventually achieve an accuracy of 94.8% in artificial neural network simulations. These results highlight a new approach for constructing hybrid systems coupling 2D semiconductors with functional ferroelectrics in a single device for tuning synaptic weight, optimizing circuit design, and building artificial neuromorphic computing systems.

Magnetic Exchange Mechanism and Quantized Anomalous Hall Effect in Bi2Se3 Film with a CrWI6 Monolayer.

Magnetizing the surface states of topological insulators without damaging their topological features is a crucial step for realizing the quantum anomalous Hall (QAH) effect and remains a challenging task. The TI-ferromagnetic material interface system was constructed and studied by the density functional theory (DFT). A two-dimensional magnetic semiconductor CrWI6 has been proven to effectively magnetize topological surface states (TSSs) via the magnetic proximity effect. The non-trivial phase was identified in the Bi2Se3 (BS) films with six quantum layers (QL) within the CrWI6/BS/CrWI6 heterostructure. BS thin films exhibit the generation of spin splitting near the TSSs, and a band gap of approximately 2.9 meV is observed at the Γ in the Brillouin zone; by adjusting the interface distance of the heterostructure, we increased the non-trivial band gap to 7.9 meV, indicating that applying external pressure is conducive to realizing the QAH effect. Furthermore, the topological non-triviality of CrWI6/6QL-BS/CrWI6 is confirmed by the nonzero Chern number. This study furnishes a valuable guideline for the implementation of the QAH effect at elevated temperatures within heterostructures comprising two-dimensional (2D) magnetic monolayers (MLs) and topological insulators.

Continuing challenges in 2D semiconductors

Continuing challenges in 2D semiconductors

Two-Dimensional Semiconductors for State-of-the-Art Complementary Field-Effect Transistors and Integrated Circuits.

As the trajectory of transistor scaling defined by Moore's law encounters challenges, the paradigm of ever-evolving integrated circuit technology shifts to explore unconventional materials and architectures to sustain progress. Two-dimensional (2D) semiconductors, characterized by their atomic-scale thickness and exceptional electronic properties, have emerged as a beacon of promise in this quest for the continued advancement of field-effect transistor (FET) technology. The energy-efficient complementary circuit integration necessitates strategic engineering of both n-channel and p-channel 2D FETs to achieve symmetrical high performance. This intricate process mandates the realization of demanding device characteristics, including low contact resistance, precisely controlled doping schemes, high mobility, and seamless incorporation of high- κ dielectrics. Furthermore, the uniform growth of wafer-scale 2D film is imperative to mitigate defect density, minimize device-to-device variation, and establish pristine interfaces within the integrated circuits. This review examines the latest breakthroughs with a focus on the preparation of 2D channel materials and device engineering in advanced FET structures. It also extensively summarizes critical aspects such as the scalability and compatibility of 2D FET devices with existing manufacturing technologies, elucidating the synergistic relationships crucial for realizing efficient and high-performance 2D FETs. These findings extend to potential integrated circuit applications in diverse functionalities.

Radical n-p Conduction Switching and Significant Photoconductivity Enhancement in NbOI2 via Pressure-Modulated Peierls Distortion.

The absence of intrinsic p-type 2D layered semiconductors has hampered the development of 2D devices, particularly in complementary metal-oxide-semiconductor (CMOS) devices and integrated circuits. Developing practical p-type semiconductors and advanced modulation techniques for precise carrier control is paramount to advancing electronic devices and systems. Here, by applying pressure to continuously tune the Peierls distortion in NbOI2, we effectively control the polarity and concentration of carriers and significantly enhance its photoelectric properties. The results demonstrate that by suppressing the off-center displacement of Nb atoms along the in-plane b direction under pressure, NbOI2 undergoes a semiconductor-to-semiconductor phase transition from C2 to C2/m, leading to a significant transition from n-type to p-type carrier behavior. Additionally, the gradual inhibition of internal interactions within Nb-Nb dimers along the in-plane c direction under high pressure facilitates electron delocalization, substantially enhancing the photoelectric properties. The photocurrent is increased by more than 3 orders of magnitude under xenon irradiation, and the spectral response range is continuously red-shifted and extended to 1450 nm. These findings highlight the potential of pressure engineering to adjust photoelectric properties effectively and flexibly, offering valuable insights for designing high-performance p-type two-dimensional semiconductors.

Cation-eutaxy-enabled III-V-derived van der Waals crystals as memristive semiconductors.

Novel two-dimensional semiconductor crystals can exhibit diverse physical properties beyond their inherent semiconducting attributes, making their pursuit paramount. Memristive properties, as exemplars of these attributes, are predominantly manifested in wide-bandgap materials. However, simultaneously harnessing semiconductor properties alongside memristive characteristics to produce memtransistors is challenging. Herein we prepared a class of semiconducting III-V-derived van der Waals crystals, specifically the HxA1-xBX form, exhibiting memristive characteristics. To identify candidates for the material synthesis, we conducted a systematic high-throughput screening, leading us to 44 prospective III-V candidates; of these, we successfully synthesized ten, including nitrides, phosphides, arsenides and antimonides. These materials exhibited intriguing characteristics such as electrochemical polarization and memristive phenomena while retaining their semiconductive attributes. We demonstrated the gate-tunable synaptic and logic functions within single-gate memtransistors, capitalizing on the synergistic interplay between the semiconducting and memristive properties of our two-dimensional crystals. Our approach guides the discovery of van der Waals materials with unique properties from unconventional crystal symmetries.

Twist-angle-tunable spin texture in WSe2/graphene van der Waals heterostructures.

Twist engineering has emerged as a powerful approach for modulating electronic properties in van der Waals heterostructures. While theoretical works have predicted the modulation of spin texture in graphene-based heterostructures by twist angle, experimental studies are lacking. Here, by performing spin precession experiments, we demonstrate tunability of the spin texture and associated spin-charge interconversion with twist angle in WSe2/graphene heterostructures. For specific twist angles, we detect a spin component radial with the electron's momentum, in addition to the standard orthogonal component. Our results show that the helicity of the spin texture can be reversed by twist angle, highlighting the critical role of the twist angle in the spin-orbit properties of WSe2/graphene heterostructures and paving the way for the development of spin-twistronic devices.