Two-dimensional Heterostructures Research Articles

Synthesis, Modulation, and Application of Two-Dimensional TMD Heterostructures.

Two-dimensional (2D) transition metal dichalcogenide (TMD) heterostructures have attracted a lot of attention due to their rich material diversity and stack geometry, precise controllability of structure and properties, and potential practical applications. These heterostructures not only overcome the inherent limitations of individual materials but also enable the realization of new properties through appropriate combinations, establishing a platform to explore new physical and chemical properties at micro-nano-pico scales. In this review, we systematically summarize the latest research progress in the synthesis, modulation, and application of 2D TMD heterostructures. We first introduce the latest techniques for fabricating 2D TMD heterostructures, examining the rationale, mechanisms, advantages, and disadvantages of each strategy. Furthermore, we emphasize the importance of characteristic modulation in 2D TMD heterostructures and discuss some approaches to achieve novel functionalities. Then, we summarize the representative applications of 2D TMD heterostructures. Finally, we highlight the challenges and future perspectives in the synthesis and device fabrication of 2D TMD heterostructures and provide some feasible solutions.

Slip Opacity and Fast Osmotic Transport of Hydrophobes at Aqueous Interfaces with Two-Dimensional Materials.

We investigate the interfacial transport of water and hydrophobic solutes on van der Waals bilayers and heterostructures formed by stacking graphene, hBN, and MoS2 using extensive ab initio molecular dynamics simulations. We compute water slippage and the diffusio-osmotic transport coefficient of hydrophobic particles at the interface by combining hydrodynamics and the theory of the hydrophobic effect. We find that slippage is dominated by the layer that is in direct contact with water and only marginally altered by the second layer, leading to a so-called "slip opacity". The screening of the lateral forces, where the liquid does not feel the forces coming from the second nearest layer, is one of the factors leading to the "slip opacity" in our systems. The diffusio-osmotic transport of small hydrophobes (with a radius below 2.5 Å) is also affected by the slip opacity, being dramatically enhanced by slippage. Furthermore, the direction of diffusio-osmotic flow is controlled by the solute size, with the flow in the opposite direction of the concentration gradient for smaller hydrophobes, and vice versa for larger ones. We connect our findings to the wetting properties of two-dimensional materials, and we propose that slippage and wetting can be controlled separately: whereas the slippage is mostly determined by the layer in closer proximity to water, wetting can be finely tuned by stacking different two-dimensional materials. Our study advances the computational design of two-dimensional materials and van der Waals heterostructures, enabling precise control over wetting and slippage properties for applications in coatings and water purification membranes.

A universal approach of constructing two-dimensional transition metal phosphide heterostructures and the superior performance in (urea-rich) water electrolysis

Transition metal phosphides have consistently shown excellent performance in electrocatalytic hydrogen production, but challenges remain in the development of efficient diverse catalysts. We designed a superhydrophilic two-dimensional (2D) TMP heterostructure with generalized approach. The phosphidation of layered double hydroxides (LDHs) with controlled exfoliation yielded multiple phosphide heterostructures (Ni2P/CoP, CoP/Fe2P and Ni2P/Fe2P) of less than 5 nm, the thinnest phosphide sheet up to date. All 2D phosphide heterojunctions are superhydrophilic (θw=10°), which is more favourable for bubble desorption. Both XPS and XANES, among other characterizations, indicated the much stronger electron-donating effect of metal sites comparing with bulk analogues, resulting in the formation of highly electron-rich P sites for facilitated proton adsorption. 2D Ni2P/CoP showed superior activity (η10=46.1 mV) for hydrogen production. Interestingly, 2D Ni2P/CoP was also highly active (η50=112.1 mV) for urea oxidation. This work demonstrates a versatile method for building superhydrophilic 2D phosphides, opening up exciting opportunities for multifunctional applications.

Ferroelectric field effect transistors based on two-dimensional CuInP2S6 (CIPS) and graphene heterostructures

Ferroelectric field effect transistors based on two-dimensional CuInP2S6 (CIPS) and graphene heterostructures

Flip-Chip-Based Fast Inductive Parity Readout of a Planar Superconducting Island

The properties of superconducting devices depend sensitively on the parity (even or odd) of the quasiparticles that they contain. Encoding quantum information in the parity degree of freedom is central in several emerging solid-state qubit architectures, including in hybrid superconductor-semiconductor devices. In the latter case, accurate, nondestructive, and time-resolved parity measurements are a challenging issue. Here, we report on control and real-time parity measurement in a superconducting island embedded in a superconducting loop and realized in a hybrid two-dimensional heterostructure using a microwave resonator. To avoid microwave losses impeding time-resolved measurements, the device and readout resonator are located on separate chips, connected via flip-chip bonding, and couple inductively through vacuum. The superconducting resonator detects the parity-dependent circuit inductance, allowing for fast parity readout. We have resolved even- and odd-parity states with a signal-to-noise ratio of SNR≈3 for an integration time of 20μs and a detection fidelity exceeding 98%. The real-time parity measurement shows a state lifetime extending into the millisecond range. Our approach will lead to a better understanding of coherence-limiting mechanisms in superconducting quantum hardware and help to advance inductive-readout schemes for hybrid qubits. Published by the American Physical Society 2024

Coupled topological edge and corner states in two-dimensional phononic heterostructures with nonsymmorphic symmetries

The exciting discovery of topological phononic states has aroused great interest in the field of acoustic wave control. However, conventional topological edge states and corner states localized at the interface and corner of the two-phase domain wall structures are limited by single channel transmission characteristics, which decreases the flexibility of designing multi-channel acoustic wave devices. Here, we propose a two-dimensional (2D) topological phononic heterostructure with nonsymmorphic symmetries to realize the multiple interface topological multimode interference effect based on the coupling of topological edge and corner states. Topological phase transitions are achieved by altering the rotation angle of the split-ring scatterers in a square lattice. The coupled edge states are generated by the coupling between the edge states of ordinary-topological-ordinary (OTO) interfaces. Moreover, the higher-order topology of the square phononic crystals (PCs) is characterized by nontrivial bulk polarization, the topological and coupled corner states splitting into two pairs appear in the square OTO bend structure owing to the nonsymmorphic PC lack of mirror symmetries. Finally, the topological robustness of the multimode interference effect of coupled edge and corner states against defects is demonstrated. Our results pave the way for guiding and trapping acoustic waves in topological nonsymmorphic heterostructures, whose multi-channel transmission capability can be employed for designing topological phononic filters, couplers and multiplexers.

LiFe0.3Mn0.7PO4-on-MXene heterostructures as highly reversible cathode materials for Lithium-ion batteries

Two-dimensional (2D) heterostructure materials, incorporating the collective strengths and synergetic properties of individual building blocks, have attracted great interest as a novel paradigm in electrode materials science. The family of 2D transition metal carbides and nitrides (e.g., MXenes) has become an appealing platform for fabricating functional materials with strong application performance. Herein, a 2D LiFe0.3Mn0.7PO4 (LFMP)–on-MXene heterostructure composite is prepared through an electrostatic self-assembly procedure. The functional groups on the surface of MXenes possess highly electronegative properties that facilitate the incorporation of LFMPs into MXenes to construct heterostructure composites. The special heterostructure of nanosized-LiFe0.3Mn0.7PO4 and MXene provides rapid Li+ and electron transport in the cathodes. This LiFe0.3Mn0.7PO4-3.0 wt% MXene composite can exhibit an excellent rate capability of 98.3 mAh g−1 at 50C and a very stable cycling performance with a capacity retention of 94.3 % at 5C after 1000 cycles. Furthermore, NaFe0.3Mn0.7PO4-3.0 wt% MXene with stable cyclability can be obtained by an electrochemical conversion method with LiFe0.3Mn0.7PO4-3.0 wt% MXene. Ex-situ XRD suggests that LiFe0.3Mn0.7PO4-on-MXene achieves a highly reversible structural evolution with a solid solution phase transformation (LFMP→LixFe0.3Mn0.7PO4 (LxFMP), LxFMP→LFMP) and a two-phase reaction (LxFMP←→Fe0.3Mn0.7PO4 (FMP)). This work provides a new direction for the use of MXenes to fabricate 2D heterostructures for lithium-ion batteries.

Modulation of Electrostatic Potential in 2D Crystal Engineered by an Array of Alternating Polar Molecules.

The moiré potential in rotationally misfit two-dimensional (2D) heterostructures has been used to build artificial exciton and electron lattices, which have become platforms for realizing exotic electronic phases. Here, we demonstrate a different approach to create a superlattice potential in 2D crystals by using the near field of an array of polar molecules. A bilayer of titanyl phthalocyanine (TiOPc), consisting of alternating out-of-plane dipoles, is deposited on monolayer MoS2. Time-resolved two-photon photoemission spectroscopy reveals a pair of interlayer exciton states with an energy difference of ∼0.1 eV, which is consistent with the electrostatic potential modulation induced by the TiOPc bilayer as determined by density functional theory calculations. Because the symmetry and the period of this potential superlattice can be changed readily by using molecules of different shapes and sizes, molecule/2D heterostructures can be promising platforms for designing artificial exciton and electron lattices.

(Invited) Manipulating Interfacial Electrochemistry with Moiré Superlattices

At electrode–electrolyte interfaces, crystallographic defects are frequently implicated as active sites that mediate interfacial electron transfer (ET) by introducing high densities of localized electronic states (DOS). However, conventional defects are challenging to deterministically synthesize and control at an atomic level, hindering the direct study of how electronic localization impacts interfacial reactivity. Azimuthal misalignment of atomically thin layers produces moiré superlattices and alters the electronic band structure, in a manner that is systematically dependent on the interlayer twist angle. Using van der Waals nanofabrication of two-dimensional heterostructures, scanning electrochemical cell microscopy measurements, and four-dimensional scanning transmission electron microscopy, we report a strong twist angle dependence of heterogeneous charge transfer kinetics at twisted bilayer and trilayer graphene electrodes with the greatest enhancement observed near the ‘magic angles’. These effects are driven by the angle-dependent engineering of moiré flat bands that dictate the electron transfer processes with the solution-phase redox couple, and the structure of the relaxed moiré superlattice. Moiré superlattices therefore serve as an unparalleled platform for systematically interrogating and exploiting the dependence of interfacial ET on local electronic structure.

Thermal and electronic properties of borophene in two-dimensional lateral graphene-borophene heterostructures empowered by machine-learning approach

Due to its electron-deficient character and complicated banding mechanism, two-dimensional (2D) borophene has gain more attentions recently. The polymorphism of 2D borophene provides more room for forming the 2D lateral heterostructures. However, the effects of confined circumstance on many properties of the borophene are unknown. In this work, using first-principles calculations and molecular simulations based on the machine learning interatomic potential, we investigated the electronic structures and thermal properties of 2D lateral graphene-borophene heterostructures (GBHs). We found that the graphene-borophene heterostructure promotes the thermal transport of borophene, and different concentrations of borophene hardly affect the lattice thermal conductivity of the interface. We also found the charge transfer induced thermal conductivity change in borophene domain of GBH, compared with that of bulk borophene. The present results not only advance our understanding of 2D lateral GBHs, but also provide valuable information on the heat transfer properties of 2D heterogeneous graphene-based materials in practical applications.

Unveiling the Synergistic Effect of Two-Dimensional Heterostructure NiFeP@FeOOH as Stable Electrocatalyst for Oxygen Evolution Reaction

Introducing multiple active sites and constructing a heterostructure are efficient strategies to develop high-performance electrocatalysts. Herein, two-dimensional heterostructure NiFeP@FeOOH nanosheets supported by nickel foam (NF) are prepared by a hydrothermal–phosphorization–electrodeposition process. The synthesis of self-supporting heterostructure NiFeP@FeOOH nanosheets on NF increases the specific surface region, while bimetallic phosphide realizes rapid charge transfer, improving the electron transfer rate. The introduction of FeOOH and the construction of a heterostructure result in a synergistic effect among the components, and the surface-active sites are abundant. In situ Raman spectroscopy showed that the excellent oxygen evolution reaction (OER) performance was due to reconstruction-induced hydroxyl oxide, which achieved a multi-active site reaction. The NiFeP@FeOOH/NF electrocatalytic activity was then significantly improved. The findings indicate that in a 1.0 M KOH alkaline solution, NiFeP@FeOOH/NF showed an OER overpotential of 235 mV at 100 mA cm−2, a Tafel slope of 46.46 mV dec−1, and it worked stably at 50 mA cm−2 for 80 h. This research proves that constructing heterostructure and introducing FeOOH are of great significance to the study of the properties of OER electrocatalysts.

Excitons in two-dimensional materials and heterostructures: Optical and magneto-optical properties

Excitons in two-dimensional materials and heterostructures: Optical and magneto-optical properties

Two-Dimensional SnSe2(1-x)S2x/MoTe2 Antiambipolar Transistors with Composition Modulation for Multivalued Inverters.

Two-dimensional (2D) van der Waals heterostructures that embody the electronic characteristics of each constituent material have found extensive applications. Alloy engineering further enables the modulation of the electronic properties in these structures. Consequently, we envisage the construction and modulation of composition-dependent antiambipolar transistors (AATs) using van der Waals heterostructures and alloy engineering to advance multivalued inverters. In this work, we calculate the electron structures of SnSe2(1-x)S2x alloys and determine the energy band alignment between SnSe2(1-x)S2x and 2H-MoTe2. We present a series of vertical AATs based on the SnSe2(1-x)S2x/MoTe2 type-III van der Waals heterostructure. These transistors exhibit composition-dependent antiambipolar characteristics through the van der Waals heterostructure, except for the SnSe2/MoTe2 transistor. The peak current (Ipeak) decreases from 43 nA (x = 0.25) to 0.8 nA (x = 1) at Vds = -2 V, while the peak-to-valley current ratio (PVR) increases from 4.5 (x = 0.25) to 6.7 × 103 (x = 1) with a work window ranging from 30 to 47 V. Ultimately, we successfully apply several specific SnSe2(1-x)S2x/MoTe2 devices in binary and ternary logic inverters. Our results underscore the efficacy of alloy engineering in modulating the characteristics of AATs, offering a promising strategy for the development of multivalued logic devices.

Tunable Band Alignment and Optoelectronic Structure of Cu2Se/PtX2 (X=Se,Te) Heterostructures: under Strain and Electric Field

Two-dimensional (2D) material-based van der Waals heterostructures (vdWHs) are an emerging strategy for regulating the electronic properties of two-dimensional materials. This study investigated the effects of a vertical electric field and strain on the band alignment transition and optoelectronic properties of Cu2Se/PtSe2 (PtTe2) vdWHs using density functional theory calculations. Cu2Se/PtSe2 vdWHs exhibited type-I to type-II band alignment transformation under an in-plane strain, interlayer distance, and electric field of 2%, 2.40 Å, and -0.40 V/Å, respectively. Moreover, Cu2Se/PtTe2 vdWH demonstrated a rich transition of types I, II, and III band alignments with in-plane strain. The absorption spectra of Cu2Se/PtSe2 (PtTe2) vdWHs show a certain degree of blue and red shift phenomenon upon the application of strain and electric fields. The predicted power conversion efficiencies at 3% in-plane strain with an interlayer distance of 3.40 Å for Cu2Se/PtSe2 (PtTe2) vdWHs were observed to be as high as 22.58% (19.80%). Thus, our work demonstrated that both strain and electric field can effectively and flexibly control the electronic properties of Cu2Se/PtSe2 (PtTe2) vdWHs, making them a promising candidate material for applications in photodetectors and field-effect transistors.

Study on interface characteristics and electron spin manipulation of two-dimensional [formula omitted] (001) heterostructures

Among numerous strongly correlated electron materials, cobalt ions in perovskite oxide SrCoO3 (SCO) possess rich valence states, playing a crucial role in determining the functionality of the material. Meanwhile, thin film preparation techniques such as epitaxial growth and pulsed laser deposition have accelerated the research progress of two-dimensional SCO films. In this study, we primarily investigate the interface characteristics and electron spin of two-dimensional SrCoO3/LaAlO3 (SCO/LAO) (SCO/LAO) (001) heterostructures using density functional theory, revealing the influence of atomic layer thickness and doping on the heterostructure. Our research results indicate that SCO/LAO heterostructures exhibit metallic behavior and ferromagnetism, and doping with oxygen vacancies and Nb2+ ions leads to significant changes in electronic properties and magnetism of the heterostructure. These findings provide insights for the experimental synthesis of two-dimensional heterostructures and suggest that two-dimensional SCO/LAO (001) heterostructures possess rich interface properties with tremendous potential applications in electronic devices.

Synergetic interface engineering and space-confined effect in CoSe2@Ti3C2Tx heterostructure for high power and long life sodium ion capacitors

The intriguing characteristics of two-dimensional (2D) heterostructures stem from their unique interfaces, which can improve ion storage capability and rate performance. However, there are still challenges in increasing the proportion of heterogeneous interfaces in materials and understanding the complex interaction mechanisms at these interfaces. Here, we have successfully synthesized confined CoSe2 within the interlayer space of Ti3C2Tx through a simple solvothermal method, resulting in the formation of a superlattice-like heterostructures of CoSe2@Ti3C2Tx. Both density functional theory (DFT) calculations and experimental results show that compared with CoSe2 and Ti3C2Tx, CoSe2@Ti3C2Tx can significantly improve adsorption of Na+ ions, while maintaining low volume expansion and high Na+ ions migration rate. The heterostructure formed by MXene and CoSe2 is a Schottky heterostructure, and its interfacial charge transfer induces a built-in electric field that promotes rapid ion transport. When CoSe2@Ti3C2Tx was used as an anode material, it exhibits a high specific capacity of up to 600.1 mAh/g and an excellent rate performance of 206.3 mAh/g at 20 A/g. By utilizing CoSe2@Ti3C2Tx as the anode and activated carbon (AC) as the cathode, the sodium-ion capacitor of CoSe2@Ti3C2Tx//AC exhibits excellent energy and power density (125.0 Wh kg−1 and 22.5 kW kg−1 at 300.0 W kg−1 and 37.5 Wh kg−1, respectively), as well as a long service life (86.3 % capacity retention over 15,300 cycles at 5 A/g), demonstrating its potential for practical applications.

A Universal In-Situ Interfacial Growth Strategy for Various MXene-Based van der Waals Heterostructures with Uniform Heterointerfaces: The Efficient Conversion from 3D Composite to 2D Heterostructures.

Two-dimensional (2D) van der Waals heterostructures endow individual 2D material with the novel functional structures, intriguing compositions, and fantastic interfaces, which efficiently provide a feasible route to overcome the intrinsic limitations of single 2D components and embrace the distinct features of different materials. However, the construction of 2D heterostructures with uniform heterointerfaces still poses significant challenges. Herein, a universal in-situ interfacial growth strategy is designed to controllably prepare a series of MXene-based tin selenides/sulfides with 2D van der Waals homogeneous heterostructures. Molten salt etching by-products that are usually recognized as undesirable impurities, are reasonably utilized by us to efficiently transform into different 2D nanostructures via in-situ interfacial growth. The obtained MXene-based 2D heterostructures present sandwiched structures and lamellar interlacing networks with uniform heterointerfaces, which demonstrate the efficient conversion from 3D composite to 2D heterostructures. Such 2D heterostructures significantly enhance charge transfer efficiency, chemical reversibility, and overall structural stability in the electrochemical process. Taking 2D-SnSe2/MXene anode as a representative, it delivers outstanding lithium storage performance with large reversible capacities and ultrahigh capacity retention of over 97% after numerous cycles at 0.2, 1.0, and 10.0 Ag-1 current density, which suggests its tremendous application potential in lithium-ion batteries.

Quasi-1D Moiré superlattices in self-twisted two-allotropic antimonene heterostructures.

Two-dimensional heterostructures, characterized by a twist angle between individual sublayers, offer unique and tunable properties distinct from standalone layers. These structures typically introduce a realm of exotic quantum phenomena due to the appearance of new, long range periodicities associated with Moiré superlattices. Using molecular beam epitaxy, we demonstrate the growth of bi-allotropic 2D-Sb heterostructures on a W(110) substrate composed of twisted α (α-Sb) and β (β-Sb) phases of antimonene. Due to the relatively weak interaction between sublayers, the twist angle is intrinsically determined for each heterostructure, revealing its inherent self-twisted nature. The different atomic lattice symmetries of both allotropes lead to the formation of distinctive quasi-1D Moiré superlattices, while the random nature of the twist angle allows for a wide modulation of the Moiré potential landscape. The observed Moiré patterns on β-Sb/α-Sb heterostructures were compared with a simple model, revealing satisfactory agreement with the experiments and strongly validating the formation of self-twisted β-Sb/α-Sb heterostructures. The samples were characterized in situ using low energy electron microscopy and diffraction techniques providing a real-time tracking of the growth process and insight into the atomic structure of the synthesized nanostructures.

In situ grown heterostructures based on MAX-derived TaO2F nanorod and TaC (MXene) nanosheet for high-performance hybrid supercapacitors

Two-dimensional MXene-based interface heterostructures have an undeniable position in the energy storage systems, particularly supercapacitors, due to their unique microstructure and electrochemical properties. However, the selection of materials in heterogeneous structures and the assembly strategy often have often have a significant impact on the properties of heterogeneous structures. This study proposes a in situ grown strategy for constructing TaO2F@TaC nano-heterostructures consisting of one-dimensional TaO2F nanorods combined with two-dimensional TaC nanosheets derived from the precursor MAX phase. The complex etching and growth mechanism from MAX to 1D/2D TaO2F@TaC is reasonably inferred. The construction of TaO2F@TaC nano-heterostructures effectively addresses the issue of MXene to restack and improves the poor conductivity of TaO2F. Density functional theory (DFT) calculations validated the electronic structure and charge distribution at the heterostructure interface, demonstrating its capability for electron conduction and ion transport. Moreover, the hybrid supercapacitor (HSC) based on TaO2F@TaC exhibited superior energy density (153.4 μWh cm−2 at a power density of 222.2 μW cm−2) and excellent cycle stability. The prepared quasi-solid-state supercapacitor (QSC) also shows a capacitance of 457.4 mF cm−2, and retains 99.38 % capacity after 8200 cycles. This work innovatively bypasses the multi-step process of preparing MXene heterostructures and provides a new approach for developing MXene-based electrode materials to enhance the performance of hybrid supercapacitors.

Observation of In-Gap States in a Two-Dimensional CrI2/NbSe2 Heterostructure.

Low-dimensional magnetic structures coupled with superconductors are promising platforms for realizing Majorana zero modes, which have potential applications in topological quantum computing. Here, we report a two-dimensional (2D) magnetic-superconducting heterostructure consisting of single-layer chromium diiodide (CrI2) on a niobium diselenide (NbSe2) superconductor. Single-layer CrI2 nanosheets, which hold antiferromagnetic (AFM) ground states by our first-principles calculations, were epitaxially grown on the layered NbSe2 substrate. Using scanning tunneling microscopy/spectroscopy, we observed robust in-gap states spatially located at the edge of the nanosheets and defect-induced zero-energy peaks inside the CrI2 nanosheets. Magnetic-flux vortices induced by an external field exhibit broken 3-fold rotational symmetry of the pristine NbSe2 superconductor, implying the efficient modulation of the interfacial superconducting states by the epitaxial CrI2 layer. A phenomenological model suggests the existence of chiral edge states in a 2D AFM-superconducting hybrid system with an even Chern number, providing a qualitatively plausible understanding for our experimental observation.