Graphene-Enhanced Metal Transfer Printing for Strong van der Waals Contacts between 3D Metals and 2D Semiconductors

2D semiconductors have shown great potentials for ultra-short channel field-effect transistors (FETs) in next-generation electronics. However, because of intractable surface states and interface barriers, it is challenging to realize high-quality contacts with low contact resistances for both p- and n- 2D FETs. Here, a graphene-enhanced van der Waals (vdWs) integration approach is demonstrated, which is a multi-scale (nanometer to centimeter scale) and reliable (≈100% yield) metal transfer strategy applicable to various metals and 2D semiconductors. Scanning transmission electron microscopy imaging shows that 2D/2D/3D semiconductor/graphene/metal interfaces are atomically flat, ultraclean, and defect-free. First principles calculations indicate that the sandwiched graphene monolayer can eliminate gap states induced by 3D metals in 2D semiconductors. Through this approach, Schottky barrier-free contacts are realized on both p- and n-type 2D FETs, achieving p-type MoTe₂, p-type black phosphorus and n-type MoS₂ FETs with on-state current densities of 404, 1520, and 761 µA µm⁻¹, respectively, which are among the highest values reported in literature.     

DFT study on type-II photocatalyst for overall water splitting: g-GaN/C2N van der Waals heterostructure

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What Has Happened to Territory?

In the concluding article of the main section of this issue, Antoine Picon evokes the earlier meaning of territory for administrators, architects and engineers, as lands that were integrated into nations or colonies by the early modern European countries. Picon traces how 18th- and 19th-century perceptions of territory with an emphasis on administrative separation fed into an attitude of both distance and sensitivity to landscape, as exemplified by the Romantic movement in painting and literature; a heritage that continued into the 20th century in architecture with its emphasis on rationalisation. Copyright © 2010 John Wiley & Sons, Ltd.     

Laser-Induced Phase Transition and Patterning of hBN-Encapsulated MoTe2

Transition metal dichalcogenides exhibit phase transitions through atomic migration when triggered by various stimuli, such as strain, doping, and annealing. However, since atomically thin 2D materials are easily damaged and evaporated from these strategies, studies on the crystal structure and composition of transformed 2D phases are limited. Here, the phase and composition change behavior of laser-irradiated molybdenum ditelluride (MoTe₂) in various stacked geometry are investigated, and the stable laser-induced phase patterning in hexagonal boron nitride (hBN)-encapsulated MoTe₂ is demonstrated. When air-exposed or single-side passivated 2H-MoTe₂ are irradiated by a laser, MoTe₂ is transformed into Te or Mo₃Te₄ due to the highly accumulated heat and atomic evaporation. Conversely, hBN-encapsulated 2H-MoTe₂ transformed into a 1T′ phase without evaporation or structural degradation, enabling stable phase transitions in desired regions. The laser-induced phase transition shows layer number dependence; thinner MoTe₂ has a higher phase transition temperature. From the stable phase patterning method, the low contact resistivity of 1.13 kΩ µm in 2H-MoTe₂ field-effect transistors with 1T′ contacts from the seamless heterophase junction geometry is achieved. This study paves an effective way to fabricate monolithic 2D electronic devices with laterally stitched phases and provides insights into phase and compositional changes in 2D materials.     

Inhomogeneous Friction Behaviour of Nanoscale Phase Separated Layered CuInP2S6

Mechanical friction leads to wear and energy dissipation, and its control is of high importance in new-generation miniature electromechanical devices. 2D materials such as graphene are considered to be excellent solid lubricants due to their ultralow friction and have attracted considerable research interest. Unique friction properties are discovered in various other 2D materials. However, the friction of functional van der Waals materials which have potential applications in novel nanoelectronics, like ferroelectric copper indium thiophosphate, has barely been studied. Herein, the study reports on the observation of inhomogeneous friction behavior existing in copper-deficient CuInP₂S₆ (Cu_0.2In_1.26P₂S₆), which exhibits a nanoscale phase separation of polar and non-polar crystalline phases. The paraelectric In_4/3P₂S₆ phase exhibits higher friction than the ferroelectric CuInP₂S₆ phase, while phase boundaries between the two phases, interestingly, display the lowest friction. The origin of this phenomenon is attributed to different lattice strains of phases together with the presence of large strains at the nanoscale phase boundaries, which also manifests in the nonuniform tip-sample adhesion force. The findings provide new insights into nanoscale device design and wear behavior of a phase-separated van der Waals ferroelectric, which may help to reduce the power consumption of friction-exhibiting devices and extend their service life.     

Manipulating Spin Exchange Interactions and Spin-Selected Electron Transfers of 2D Metal Phosphorus Trisulfide Crystals for Efficient Oxygen Evolution Reaction

Because oxygen molecules in the ground state favor a triplet spin configuration, spin-polarized electrons at electrocatalysts may promote the generation of parallel spin-aligned oxygen atoms, enhancing oxygen evolution reaction (OER) kinetics. In this study, a significant enhancement of OER performance is demonstrated by controlling the spin-exchange interaction and spin-selected electron transfer of 2D Co_xFe_1−xPS₃ (x = 0–0.45) van der Waals (vdW) single crystals through Co doping. The pristine FePS₃ exhibits antiferromagnetic orbital ordering, while the Co-doped FePS₃ exhibits the emergence of interatomic ferromagnetism due to doping-mediated magnetic exchange interactions. The coupling between Fe and Co ions in the Co-doped FePS₃ crystal allows the formation of efficient spin-selective electron transfer channels compared to the pristine FePS₃. The correlation of spin-exchange interactions and spin-selected electron transfers of 2D Co-doped FePS₃ crystals with a superior OER performance is further revealed by superconducting quantum interference device magnetometer, in situ X-ray absorption near edge spectra and density functional theory simulations. The result suggests that manipulating the spin-exchange interactions of 2D vdW crystals to enhance the spin-selected electron transfer efficiencies through doping is an effective strategy to boost their OER catalytic performances.     

Synthesis of Ultrathin Topological Insulator β-Ag2Te and Ag2Te/WSe2-Based High-Performance Photodetector

β-Ag₂Te has attracted considerable attention in the application of electronics and optoelectronics due to its narrow bandgap, high mobility, and topological insulator properties. However, it remains a significant challenge to synthesize 2D Ag₂Te because of the non-layered structure of Ag₂Te. Herein, the synthesis of large-size, ultrathin single crystal topological insulator 2D Ag₂Te via the van der Waals epitaxial method for the first time is reported. The 2D Ag₂Te crystal exhibits p-type conduction behavior with high carrier mobility of 3336 cm² V⁻¹ s⁻¹ at room temperature. Taking advantage of the high mobility and perfect electron structure of Ag₂Te, the Ag₂Te/WSe₂ heterojunctions are fabricated via mechanical stacking and show an ultrahigh rectification ratio of 2 × 10⁵. Ag₂Te/WSe₂ photodetector also exhibits self-driven properties with a fast response speed (40 µs/60 µs) in the near-infrared region. High responsivity (219 mA W⁻¹) and light ON/OFF ratio of 6 × 10⁵ are obtained under the photovoltaic mode. The overall performance of the Ag₂Te/WSe₂ photodetector is significantly competitive among all reported 2D photodetectors. These results indicate that 2D Ag₂Te is a promising candidate for future electronic and optoelectronic applications.     

Interlayer Sodium Plating/Stripping in Van der Waals-Layered Quantum Dot Superstructure

Assembling quantum dots (QDs) into van der Waals (vdW)-layered superstructure holds great promise for the development of high-energy-density metal anode. However, designing such a superstructure remains to be challenging. Here, a chemical-vapor Oriented Attachment (OA) growth strategy is proposed to achieve the synthesis of vdW-layered carbon/QDs hybrid superlattice nanosheets (Fe₇S₈@CNS) with a large vdW gap of 3 nm. The Fe₇S₈@CNS superstructure is assembled by carbon-coated Fe₇S₈ (Fe₇S₈@C) QDs as building blocks. Interestingly, the Fe₇S₈@CNS exhibits two kinds of edge dislocations similar to traditional atom-layered materials, suggesting that Fe₇S₈@C QDs exhibit quasi-atomic growth behavior during the OA process. More interestingly, when used as host materials for sodium metal anodes, the Fe₇S₈@CNS shows the interlayer sodium plating/stripping behavior, which well suppresses Na dendrite growth. As a result, the cell with Fe₇S₈@CNS anode can keep stable cycling for 1000 h with a high Coulombic efficiency (CE) of ≈99.5% at 3.0 mA cm⁻² and 3.0 mAh cm⁻². Noticeably, the Na@Fe₇S₈@CNS||Na₃V₂(PO₄)₃ full cells can attain a capacity of 88.8 mAh g⁻¹ with a retention of 97% after 1000 cycles at 1.0 A g⁻¹ (≈8 C), showing excellent cycle stability for practical applications. This work enriches the vdW-layered QDs superstructure family and their application toward energy storage.     

Van Der Waals Heterojunction Modulated Charge Collection for H2O2 Production Photocathode

Efficient photocarrier generation and collection are highly desirable for solar-fuel conversion systems. However, the latter is challenging for many photoelectrodes due to carrier losses happening in the semiconductor-electrolyte interface and the semiconductor-substrate interface. To overcome it, a novel Au/CuBi₂O₄ (CBO)/PtSe₂ van der Waals heterojunction photocathode has been developed to boost photocarrier collection. As a result, the ohmic contact at Au/CBO interface shows a low resistance for hole transfer. Meanwhile, while promoting electron transfer, the Van der Waals heterojunction of CBO/PtSe₂ interface with energy barrier-blocked holes. Concurrently, the composite photocathode exhibits significantly enhanced performance: a photocurrent of −0.59 mA cm⁻² at 0.5 V versus reversible hydrogen electrode and an H₂O₂ generation rate of 2.39 mol (Lhm²)⁻¹. This work demonstrates the charge regulation role of Van der Waals heterojunctions in the solar-fuel system. More broadly, Van der Waals heterojunctions provide an excellent bond-free approach to creating versatile optoelectronic devices.     

Giant and Nonvolatile Control of Exchange Bias in Fe3GeTe2/Irradiated Fe3GeTe2/MgO Heterostructure Through Ultralow Voltage

The discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Recent experiments have demonstrated that the magnetic properties of van der Waals magnets can be tuned by various gate modulations, although most of them are volatile and require gate voltages no lower than several volts. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe₃GeTe₂/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented. The discovery of an ionic-irradiated phase formed in Fe₃GeTe₂ by MgO sputtering revealed that an exchange bias effect can be obtained in this heterostructure and tuned from ≈700 to 0 Oe through voltages ranging from 5 to 20 mV. Owing to the high stability of oxidized Fe₃GeTe₂, the voltage-driven oxygen incorporated into Fe₃GeTe₂ from the irradiated phase induces a nonvolatile magnetism modulation that can be retained after turning off the gate voltage. These findings demonstrate a methodology to modulate the magnetism of van der Waals magnets, opening new opportunities to fabricate all-solid, long-retention, and low-dissipation nano-electronic devices using van der Waals materials.