Two-dimensional 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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Enhanced Activity and Selectivity for Nitrogen Reduction Reaction in Electrode-based Heterostructures: A DFT Computational Study.

Developing sustainable and efficient catalysts for ammonia synthesis from atmospheric molecular N2 under ambient conditions presents a significant 21st-century challenge. Two-dimensional heterostructures, particularly single-atom catalysts (SACs) supported on two-dimensional materials, have emerged as a promising avenue due to their remarkable catalytic activity and selectivity. Electrides, characterized by an abundance of free electrons and high surface activity, have attracted substantial attention in this context. Through density functional theory (DFT) calculations, this study proposes electride-graphene heterostructures (EGHS) as catalysts to effectively regulate charge distribution at the catalytic center, facilitating the optimization of catalytic performance. The EGHS model addresses challenges related to excessive adsorbate binding, mitigating electron transfer compared to electride monolayer adsorption. This novel approach utilizes heterogeneous heterostructures to finely tune the catalytic site, optimizing electron input for enhanced catalysis.Based on the optimized charge transfer for N2 activation, the Cr-doped EGHS (Cr@EGHS) exhibits a promising performance in the nitrogen reduction reaction, leading to, a relatively low limiting potential of -0.85 V and high selectivity. The hypothesis charge transfer depend on N2 activation is further supported by modulating the distance between component layers of heterostructure. These findings contribute to design principles for 2D heterostructure catalysts and offer a reference for experimental synthesis.

In situ constructed ferroelectric-nanonet-supported heterostructure thin film for high-performance photovoltaics

Heterostructure, which combines robust ferroelectrics and narrow-bandgap semiconductors together, is promising for acquiring both intensive photocurrent and large photovoltage output in photovoltaics; therefore, manipulation of the properties by engineering heterostructure has driven significant research activity. Herein, well-known Aurivillius-type ferroelectric Bi2WO6 and mullite-type Bi2Fe4O9 are chosen, and an excellent platform to investigate the role of ferroelectric-semiconductor heterostructure in tuning the photovoltaic effect is constructed by in situ depositing p-type nanoscale Bi2Fe4O9 onto n-type Bi2WO6 nanonet matrix. The nanonet-supported two-dimensional planar-like heterostructure and its ferroelectric domain switchability are confirmed by atomic force microscopy techniques. Significantly, the desired Bi2WO6/Bi2Fe4O9 film optimizes the key steps from light to electricity, and exhibits a large and stable photovoltaic effect, achieving four/three orders of magnitude enhancement of short-circuit photocurrent density/open-circuit voltage under laser irradiation. Furthermore, electric-field-controlled switchable asymmetric photoresponse and device stability were clearly observed, due to, in particular, Bi2WO6/Bi2Fe4O9 interfacial Schottky barrier formation and its modulation by poling-modified ferroelectric polarization. These findings highlight an important insight to construct diversified heterostructures with multi-functioning performances.

Two-dimensional heterostars from nanographene and group-III nitrides for efficient spin filters: Spintronic and quantum transport properties

Two-dimensional (2D) heterostructures open the door toward novel applications, such as multiterminal transistors, by integrating the properties of the participant materials. Here we construct 2D heterostars from zigzag-triangular graphene (ZTG) and group-III nitrides. They are investigated using density functional theory and non-equilibrium Green’s function approaches. These heterostars are ferromagnetically spin-ordered devices that originate and distribute on ZTG. Further control over the net spin can be achieved by forming bilayer heterostars where spin can be increased, decreased, or removed. Single-layer heterostars are wide energy gap semiconductors of ∼3.5 eV that decrease to ∼1 eV in bilayers or are kept unchanged in other bilayers. Quantum transport calculations show that different spintronic devices can be constructed by connecting the source/drain voltage to different terminals of the nanostars. These devices are characterized by a low tunneling current through the potential barrier of group-III nitrides that significantly increased by applying gate voltage. A spin filter based on graphene-AlN nanostar with efficient separation of spin-up current is demonstrated. Also, a significant improvement in both the spin-up and spin-down currents can be achieved in the same nanostar but using other terminals. 2D heterostars are then potential candidates for building multi-terminal spintronic devices.

Chiral limit and origin of topological flat bands in twisted transition metal dichalcogenide homobilayers

The observation of zero field fractional quantum Hall analogs in twisted transition metal dichalcogenides (TMDs) asks for a deeper understanding of what mechanisms lead to topological flat bands in two-dimensional heterostructures, and what makes TMDs an excellent platform for topologically ordered phases, surpassing twisted bilayer graphene. To this aim, we explore the chiral limits of massive Dirac theories applicable to C3-symmetric moiré materials, and show their relevance for both bilayer graphene and TMD homobilayers. In the latter, the Berry curvature of valence bands leads to relativistic corrections of the moiré potential that promote band flattening, and permit a limit with exactly flat bands with nonzero Chern number. The relativistic corrections enter as a layer-orbit coupling, analogous to spin-orbit coupling for relativistic Dirac fermions, which we show is non-negligible on the moiré scale. The Berry curvature of the TMD monolayers therefore plays an essential role in the flattening of moiré Chern bands in these heterostructures.

Directional Pumpless Transport of Biomolecules through Self-Propelled Nanodroplets on Patterned Heterostructures.

The surface patterning in natural systems has exhibited appreciable functional advantages for life activities, which serve as inspiration for the design of artificial counterparts to achieve functions such as directional liquid transport at the nanoscale. Here, we propose a patterned two-dimensional (2D) in-plane heterostructure with a triangle-shaped hexagonal boron nitride (hBN) track embedded in graphene nanosheets, which can achieve unidirectional and self-propelled transport of nanodroplets carrying various biomolecules such as DNA, RNA, and peptides. Our extensive MD simulations show that the wettability gradient on the patterned heterostructure can drive the motion of nanodroplet with an instantaneous acceleration, which also permits long-distance transport (>100 nm) at the microsecond time scale. The different behaviors of various types of biomolecules have been further studied systematically within the transporting nanodroplets. These findings suggest that these specially designed, patterned heterostructures have the potential for spontaneous, directional transport of important biomolecules, which might be useful in biosensing, drug delivery, and biomedical nanodevices.

Significantly improved optoelectronic performances of two-dimensional WS2(Er3+)/WSe2(Er3+) van der waals heterojunctions

Two-dimensional heterostructures with excellent properties are potential for high performance optoelectronic applications, since the ultimate thickness limit is possible to be broken with the advent of atomically thin materials. Yet the studies have also been largely constrained by the low physical doping space and absorption rates due to the atomic-thickness layers, and the efforts for high carrier concentrations and gain remain a significant challenge. Herein, high concentration of Er3+ ions are in-situ doped in WS2 and WSe2 atomic layers by chemical vapor deposition (CVD) method. The prepared layers are characterized by atomic force microscopy, transmission electron microscopy, photoluminescence (PL) spectroscopy, and Raman spectroscopy, respectively. 13–14 at% of Er3+ doping concentration in WSe2 and WS2 are examined by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) with high consistency. A microregion fixed-point transfer technique is used to transfer the WS2(Er3+) monolayers onto the WSe2(Er3+) monolayers to form vertical van der Waals heterojunctions. Excellent performances are measured from the WS2(Er3+)/WSe2(Er3+) photodetector with the photoresponsivity of up to 67.4 A/W, external quantum efficiency of 13,202 %, and detectivity of 7.07 × 1011 Jones. Our results prove the effectiveness of Er3+ in-situ doping in WS2(Er3+)/WSe2(Er3+) heterojunctions for high-performance photodetectors.

Proximity-induced gapless superconductivity in two-dimensional Rashba semiconductor in magnetic field

Two-dimensional semiconductor-superconductor heterostructures form the foundation of numerous nanoscale physical systems. However, measuring the properties of such heterostructures, and characterizing the semiconductor in-situ is challenging. A recent experimental study by [Phys. Rev. Lett. 128, 107701 (2022)] was able to probe the semiconductor within the heterostructure using microwave measurements of the superfluid density. This work revealed a rapid depletion of superfluid density in semiconductor, caused by the in-plane magnetic field which in presence of spin-orbit coupling creates so-called Bogoliubov Fermi surfaces. The experimental work used a simplified theoretical model that neglected the presence of non-magnetic disorder in the semiconductor, hence describing the data only qualitatively. Motivated by experiments, we introduce a theoretical model describing a disordered semiconductor with strong spin-orbit coupling that is proximitized by a superconductor. Our model provides specific predictions for the density of states and superfluid density. Presence of disorder leads to the emergence of a gapless superconducting phase, that may be viewed as a manifestation of Bogoliubov Fermi surface. When applied to real experimental data, our model showcases excellent quantitative agreement, enabling the extraction of material parameters such as mean free path and mobility, and estimating gg-tensor after taking into account the orbital contribution of magnetic field. Our model can be used to probe in-situ parameters of other superconductor-semiconductor heterostructures and can be further extended to give access to transport properties.

Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction.

Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron-phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe2. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon-phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material's widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.

Two-Dimensional Lanthanide Metal-Organic Framework Heterostructures for Noninvasively Photoresponsive High-Security Photonic Barcodes.

Stimuli-responsive micro/nanoscale photonic barcodes show great capacity for encryption and anticounterfeiting technologies due to multiple authentications, yet their application is commonly restricted by invasive stimuli. Herein, we report noninvasive light-stimulated high-security photonic barcodes based on spatially assembled photoresponsive two-dimensional (2D) 1,3,5-benzenetribenzoate (BTB)@Ln-MOF host-guest heterostructures. The photoluminescence (PL) spectra information on BTB@Ln-MOF heterostructures could be precisely controlled by the different wavelengths of ultraviolet (UV) light trigger. By using the PL properties and 2D heterostructures as cryptographic primitives, spatially resolved smart photonic barcodes based on both spectral and graphical coding are realized in BTB@Ln-MOF host-guest materials. These results will pave an avenue for the development of smart stimuli-responsive photonic barcodes for anticounterfeiting applications.

Two-dimensional van der Waals stack heterostructures for flexible thermoelectrics

The incorporation of disparate materials into heterostructures has arisen as a formidable technique for modulating interfaces and electronic configurations. The introduction of two-dimensional (2D) materials has unveiled unparalleled prospects for generating innovative heterostructures in the guise of van der Waals stacks. Tantalum sulfide (TaS2), a prominent 2D material, has been extensively studied across various domains, faces constraints in thermoelectric conversion attributed to its diminished absolute Seebeck coefficient below 10 μV K−1. By constructing a two-dimensional van der Waals stack heterostructure, TaS2/organics/TiS2, a significantly enhanced absolute Seebeck coefficient of 38.3 μV K−1 was obtained, mostly attributed to the induced interfacial effect. The power factor reached 87.6 μW m−1 K−2, marking a sevenfold increase compared to the original. The thermoelectric generator demonstrated a maximum power of 86.4 nW at a temperature difference of 40 K. Employing such heterostructure films in tactile and respiration sensors demonstrated encouraging prospects for aiding the visually impaired with language assistance and facilitating real-time monitoring of respiratory rates for health monitoring purposes. This study highlights the expansive potential of two-dimensional van der Waals stack heterostructure technology for use in flexible thermoelectric generators, wearable sensors, and beyond.

Two-dimensional lead halide perovskite lateral homojunctions enabled by phase pinning

Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.

Charge Critical Phenomena in a Field Heterostructure with Two-Dimensional Crystal

The charge properties and regularities of mutual influence of the electro-physical parameters in a metal (M)/insulator (I)/two-dimensional crystal heterostructure were studied. In one case, the transition metal dichalcogenide (TMD) MoS2 was considered as a two-dimensional crystal, and in another the Weyl semi-metal (WSM) ZrTe5, representative of a quasi-two-dimensional crystal was chosen for this purpose. By self-consistently solving the electrostatic equations of the heterostructures under consideration and the Fermi–Dirac distribution, the relationship between such parameters as the concentration of charge carriers, chemical potential, and quantum capacitance of the TMD (WSM), as well as the capacitance of the I layer and the interface capacitance I–TMD (WSM), and their dependence on the field electrode potential, have been derived. The conditions for the emergence of charge instability and the critical phenomena caused by it are also determined.

Electrically induced directional ion migration in two-dimensional perovskite heterostructures

Electrically induced directional ion migration in two-dimensional perovskite heterostructures

Strain-Induced Self-Assembly at Interface of Two-Dimensional Heterostructures Boosts CO2 Reduction to Methanol by H2O.

CO2 conversion with pure H2O into CH3OH and O2 driven by solar energy can supply fuels and life-essential substances for extraterrestrial exploration. However, the effective production of CH3OH is significantly challenging. Here we report an organozinc complex/MoS2 heterostructure linked by well-defined zinc-sulfur covalent bonds derived by the structural deformation and intensive coupling of dx2-y2(Zn)-p(S) orbitals at the interface, resulting in distinctive charge transfer behaviors and excellent redox capabilities as revealed by experimental characterizations and first-principle calculations. The synthesis strategy is further generalized to more organometallic compounds, achieving various heterostructures for CO2 photoreduction. The optimal catalyst delivers a promising CH3OH yield of 2.57 mmol gcat-1 h-1 and selectivity of more than 99.5%. The reverse water gas shift mechanism is identified for methanol formation. Meanwhile, energy-unfavorable adsorption of methanol on MoS2, where the photogenerated holes accumulate, ensures the selective oxidation of water over methanol.

Prediction of large spin-valley polarization in the Janus 2H–WSSe monolayer on VN magnetic substrate

Two-dimensional heterostructures based on transition metal dichalcogenides (TMDs) exhibit broad application prospects in valleytronics due to the space-reversal symmetry breaking and strong spin–orbit coupling. In this work, the electronic structure, magnetic anisotropy and valley polarization of 2H–WSSe/VN van der Waals heterostructure under various interlayer spacings, magnetic angle and in-plane strain are investigated in detail by first-principles calculations. The stacked configuration of Se-C2-1 with most stable structure shows the largest valley polarization of 386.5[Formula: see text]meV. By adjusting the interlayer spacing of heterostructure, the largest valley polarization of 702.7[Formula: see text]meV appears in Se-C2-1 stacked configuration with interlayer spacing of 2.24 Å. The magnetic angle [Formula: see text] exhibits significant effects on valley polarization and magnetic anisotropy of 2H–WSSe/VN heterostructures. The stability and valley polarization of 2H–WSSe/VN heterostructure decrease after the in-plane biaxial strain is applied. The large and tunable valley polarization as well as magnetic anisotropy in the 2H–WSSe/VN heterostructures make it potential applications in valleytronic devices.

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.

Flux-Tunable Josephson Diode Effect in a Hybrid Four-Terminal Josephson Junction.

We investigate the direction-dependent switching current in a flux-tunable four-terminal Josephson junction defined in an InAs/Al two-dimensional heterostructure. The device exhibits the Josephson diode effect with switching currents that depend on the sign of the bias current. The superconducting diode efficiency, reaching a maximum of |η| ≈ 34%, is widely tunable─both in amplitude and sign─as a function of magnetic fluxes and gate voltages. Our observations are supported by a circuit model of three parallel Josephson junctions with nonsinusoidal current-phase relation. With respect to conventional Josephson interferometers, phase-tunable multiterminal Josephson junctions enable large diode efficiencies in structurally symmetric devices, where local magnetic fluxes generated on the chip break both time-reversal and spatial symmetries. Our work presents an approach for developing Josephson diodes with wide-range tunability that do not rely on exotic materials.

Theory of resonantly enhanced photo-induced superconductivity

Optical driving of materials has emerged as a versatile tool to control their properties, with photo-induced superconductivity being among the most fascinating examples. In this work, we show that light or lattice vibrations coupled to an electronic interband transition naturally give rise to electron-electron attraction that may be enhanced when the underlying boson is driven into a non-thermal state. We find this phenomenon to be resonantly amplified when tuning the boson’s frequency close to the energy difference between the two electronic bands. This result offers a simple microscopic mechanism for photo-induced superconductivity and provides a recipe for designing new platforms in which light-induced superconductivity can be realized. We discuss two-dimensional heterostructures as a potential test ground for light-induced superconductivity concretely proposing a setup consisting of a graphene-hBN-SrTiO3 heterostructure, for which we estimate a superconducting Tc that may be achieved upon driving the system.

MXene-mediated Interfacial Growth of 2D-2D Heterostructured Nanomaterials as Cathodes for Zn-based Aqueous Batteries.

In this study, we introduce a novel approach for synthesizing two-dimensional (2D) MXene heterostructures featuring a sandwiched and cross-linked network structure. This method addresses the common issue of activity degradation in 2D nanomaterials caused by inevitable aggregation. By utilizing the distinct surface characteristics of MXene, we successfully induced the growth of various 2D nanomaterials on MXene substrates. This strategy effectively mitigates self-stacking defects and augments the exposure of surface areas. In particular, the obtained 2D-2D MXene@NiCo-layered double hydroxide (MH-NiCo) heterostructures exhibit enhanced structural stability, improved chemical reversibility, and heightened charge transfer efficiency, outperforming pure NiCo LDH. The aqueous MH-Ni4Co1//Zn@carbon cloth (MH-Ni4Co1//Zn@CC) battery demonstrates exceptional performance with a remarkable specific capacity of 0.61 mAh cm-2, maintaining 96.6 % capacitance after 2300 cycles. Additionally, it achieves an energy density of 1.047 mWh cm-2 and a power density of 32.899 mW cm-2. This research not only paves the way for new design paradigms in energy-related nanomaterials but also offers invaluable insights for the application and optimization of 2D-2D heterostructures in advanced electrochemical devices.