Large-Area Synthesis of Ferromagnetic Fe5−xGeTe2/Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe_5−xGeTe₂/graphene heterostructures is achieved by vdW epitaxy of Fe_5−xGeTe₂ on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe_5−xGeTe₂ and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.     

A π-Conjugated Van der Waals Heterostructure Between Single-Atom Ni-Anchored Salphen-Based Covalent Organic Framework and Polymeric Carbon Nitride for High-Efficiency Interfacial Charge Separation

Semiconductor-based heterostructures have exhibited great promise as a photocatalyst to convert solar energy into sustainable chemical fuels, however, their solar-to-fuel efficiency is largely restricted by insufficient interfacial charge separation and limited catalytically active sites. Here the integration of high-efficiency interfacial charge separation and sufficient single-atom metal active sites in a 2D van der Waals (vdW) heterostructure between ultrathin polymeric carbon nitride (p-CN) and Ni-containing Salphen-based covalent organic framework (Ni-COF) nanosheets is illustrated. The results reveal a Ni N₂ O₂ chemical bonding in NiCOF nanosheets, leading to a highly separated single-atom Ni sites, which will function as the catalytically active sites to boost solar fuel production, as confirmed by X-ray absorption spectra and density functional theory calculations. Using ultrafast femtosecond transient adsorption (fs-TA) spectra, it shows that the vdW p-CN/Ni-COF heterostructure exhibits a faster decay lifetime of the exciton annihilation (τ = 18.3 ps) compared to that of neat p-CN (32.6 ps), illustrating an efficiently accelerated electron transfer across the vdW heterointerface from p-CN to Ni-COF, which thus allows more active electrons available to participate in the subsequent reduction reactions. The photocatalytic results offer a chemical fuel generation rate of 2.29 mmol g⁻¹ h⁻¹ for H₂ and 6.2 µmol g⁻¹ h⁻¹ for CO, ≈127 and three times higher than that of neat p-CN, respectively. This work provides new insights into the construction of a π-conjugated vdW heterostructure on promoting interfacial charge separation for high-efficiency photocatalysis.     

Magnetic interaction in the manganite/intermetallic compound heterostructure

We investigate hybrid magnetic heterostructures consisting of thin epitaxial films of oxide ferromagnets La_0.7Sr_0.3MnO₃ and intermetallic superlattices (TeCo₂/FeCo) _n . The magnetic characteristics of the heterostructures were studied by the ferromagnetic resonance and magneto-optical Kerr effect techniques. The interlayer interaction in a heterostructure is shown to be antiferromagnetic. It was established that the interface with the intermetallic compound broadens the ferromagnetic resonance line of the manganite film.     

SnSe/MoS2 van der Waals Heterostructure Junction Field‐Effect Transistors with Nearly Ideal Subthreshold Slope

The minimization of the subthreshold swing (SS) in transistors is essential for low‐voltage operation and lower power consumption, both critical for mobile devices and internet of things (IoT) devices. The conventional metal‐oxide‐semiconductor field‐effect transistor requires sophisticated dielectric engineering to achieve nearly ideal SS (60 mV dec^?1 at room temperature). However, another type of transistor, the junction field‐effect transistor (JFET) is free of dielectric layer and can reach the theoretical SS limit without complicated dielectric engineering. The construction of a 2D SnSe/MoS₂ van der Waals (vdW) heterostructure‐based JFET with nearly ideal SS is reported. It is shown that the SnSe/MoS₂ vdW heterostructure exhibits excellent p–n diode rectifying characteristics with low saturate current. Using the SnSe as the gate and MoS₂ as the channel, the SnSe/MoS₂ vdW heterostructure exhibit well‐behavioured n‐channel JFET characteristics with a small pinch‐off voltage V_P of ?0.25 V, nearly ideal subthreshold swing SS of 60.3 mV dec^?1 and high ON/OFF ratio over 10⁶, demonstrating excellent electronic performance especially in the subthreshold regime.     

Ultrafast optical and magneto-optical studies of III–V ferromagnetic semiconductors

Abstract

We use ultrafast optical techniques to investigate the dynamics of charge and spin carriers and coherent phonons as well as magnetic order in III-V ferromagnetic semiconductors. We observe a rich array of dynamical phenomena that are absent in traditional nonmagnetic semiconductors or metallic ferromagnets. Very short charge and spin lifetimes of the photoinjected carriers (～2ps) and multi-level charge decay dynamics are observed, which are attributed to a large density of mid-bandgap states introduced during low temperature molecular beam epitaxy (LT-MBE) growth and highly p-type Mn doping. During the very short free carrier lifetime, the coercivity of the system is seen to be reduced. We attribute this photo-induced ‘softening’ to the transient modification of carrier-mediated ferromagnetic exchange coupling between Mn spins. After the photogenerated free electrons are trapped by defects, periodic oscillations appear in differential reflectivity due to the coherent generation of acoustic phonon wavepackets.     

Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal–insulator–semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS₂, which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS₂, followed by formation and radiative recombination of excitons in WS₂. The photocarrier transfer rate can be described by Fowler–Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.     

Twinned Growth of Metal‐Free,Triazine‐Based Photocatalyst Films as Mixed‐Dimensional (2D/3D) van der Waals Heterostructures

Design and synthesis of ordered, metal‐free layered materials is intrinsically difficult due to the limitations of vapor deposition processes that are used in their making. Mixed‐dimensional (2D/3D) metal‐free van der Waals (vdW) heterostructures based on triazine (C₃N₃) linkers grow as large area, transparent yellow‐orange membranes on copper surfaces from solution. The membranes have an indirect band gap (E _g,opt = 1.91 eV, E _g,elec = 1.84 eV) and are moderately porous (124 m² g^?1). The material consists of a crystalline 2D phase that is fully sp² hybridized and provides structural stability, and an amorphous, porous phase with mixed sp²–sp hybridization. Interestingly, this 2D/3D vdW heterostructure grows in a twinned mechanism from a one‐pot reaction mixture: unprecedented for metal‐free frameworks and a direct consequence of on‐catalyst synthesis. Thanks to the efficient type I heterojunction, electron transfer processes are fundamentally improved and hence, the material is capable of metal‐free, light‐induced hydrogen evolution from water without the need for a noble metal cocatalyst (34 µmol h^?1 g^?1 without Pt). The results highlight that twinned growth mechanisms are observed in the realm of “wet” chemistry, and that they can be used to fabricate otherwise challenging 2D/3D vdW heterostructures with composite properties.     

Tunneling Diode Based on WSe2/SnS2 Heterostructure Incorporating High Detectivity and Responsivity

van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next‐generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe₂/SnS₂ photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe₂/SnS₂ vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe₂/SnS₂ vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 10¹³ Jones (I_ph/I_dark ratio of ≈10⁶) and photoresponsivity of 244 A W^?1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm^?2).     

A Robust n-n Heterojunction: Cu?N and B?N Boosting for Ambient Electrocatalytic Nitrogen Reduction to Ammonia

An n-n type heterojunction comprising with Cu N and B N dual active sites is synthesized via in situ growth of a conductive metal–organic framework (MOF) [Cu₃(HITP)₂] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets (hereafter denoted as Cu₃(HITP)₂@h-BN) for the electrocatalytic nitrogen reduction reaction (eNRR). The optimized Cu₃(HITP)₂@h-BN shows the outstanding eNRR performance with the NH₃ production of 146.2 µg h⁻¹ mg_cat⁻¹ and the Faraday efficiency of 42.5% due to high porosity, abundant oxygen vacancies, and Cu N/B N dual active sites. The construction of the n-n heterojunction efficiently modulates the state density of active metal sites toward the Fermi level, facilitating the charge transfer at the interface between the catalyst and reactant intermediates. Additionally, the pathway of NH₃ production catalyzed by the Cu₃(HITP)₂@h-BN heterojunction is illustrated by in situ FT-IR spectroscopy and density functional theory calculation. This work presents an alternative approach to design advanced electrocatalysts based on conductive MOFs.     

Controlling Magnetic and Optical Properties of the van der Waals Crystal CrCl3−xBrx via Mixed Halide Chemistry

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy—a tendency of spins to align in a certain crystallographic direction known as easy‐axis. In layered materials, two limits of easy‐axis are in‐plane (XY) and out‐of‐plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin‐orbitronic functions. Here, a unique experiment is designed to control the easy‐axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl_3?xBr_x crystals between CrCl₃ with XY and CrBr₃ with Ising anisotropy. The easy‐axis is controlled between the two limits by varying spin–orbit coupling with the Br content in CrCl_3?x Br_x. The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.     

Promoted Photocharge Separation in 2D Lateral Epitaxial Heterostructure for Visible-Light-Driven CO2 Photoreduction

Photocarrier recombination remains a big barrier for the improvement of solar energy conversion efficiency. For 2D materials, construction of heterostructures represents an efficient strategy to promote photoexcited carrier separation via an internal electric field at the heterointerface. However, due to the difficulty in seeking two components with suitable crystal lattice mismatch, most of the current 2D heterostructures are vertical heterostructures and the exploration of 2D lateral heterostructures is scarce and limited. Here, lateral epitaxial heterostructures of BiOCl @ Bi₂O₃ at the atomic level are fabricated via sonicating-assisted etching of Cl in BiOCl. This unique lateral heterostructure expedites photoexcited charge separation and transportation through the internal electric field induced by chemical bonding at the lateral interface. As a result, the lateral BiOCl @ Bi₂O₃ heterostructure demonstrates superior CO₂ photoreduction properties with a CO yield rate of about 30 µmol g⁻¹ h⁻¹ under visible light illumination. The strategy to fabricate lateral epitaxial heterostructures in this work is expected to provide inspiration for preparing other 2D lateral heterostructures used in optoelectronic devices, energy conversion, and storage fields.     

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering

2D transition‐metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS₂), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS₂. An overall control of the electronic states of VS₂ is also demonstrated: bond‐enlarging triggering a metal‐to‐semiconductor electronic transition and bond‐compression inducing metallization in 2D VS₂. The pristine VS₂ lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.     

Emergence of a Non-Van der Waals Magnetic Phase in a Van der Waals Ferromagnet

Manipulation of long-range order in 2D van der Waals (vdW) magnetic materials (e.g., CrI₃, CrSiTe₃ ,etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets that, in turn, affects the nanoelectronic /spintronic device performance. Counterintuitively, the current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr₂Te₃ (T_C2 ≈160 K) in the parent vdW magnetic semiconductor Cr₂Ge₂Te₆ (T_C1 ≈69 K). The coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magneto-transport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to the rather common poor environmental stability of the vdW magnets, the results open possibilities of finding air-stable novel materials having multiple magnetic phases.     

Hole Spin Polarization in Metallic Ferromagnetic GaMnAs Multilayers and Superlattices: Lateral and Bloch Miniband Transport

It is shown that spin-polarized currents occur in metallic and ferromagnetic Ga_1–x Mn _x As/GaAs multilayered structures, as a result of the magnetic interaction between holes and the Mn ions. The magnetic layers act as potential barriers for holes with spins aligned parallel to the layer magnetization, and as potential wells for the inverse spin polarization. In the case of currents in-plane, holes with spin parallel and antiparallel to this magnetization move in different regions. By choosing properly the magnetic and the nonmagnetic layers widths, a spin-polarized transport with a difference of an order of magnitude on the mobilities for each spin polarization is predicted to occur. Spin-polarized minibands are also shown to occur in a superlattice based on the same structure. We calculated the dependence of the spin polarization with the superlattice parameters, and we discuss how this polarization affects the Bloch miniband transport in such ferromagnetic superlattice.     

Overcoming the Limits of the Interfacial Dzyaloshinskii–Moriya Interaction by Antiferromagnetic Order in Multiferroic Heterostructures

Topologically protected magnetic states have a variety of potential applications in future spintronics owing to their nanoscale size (<100 nm) and unique dynamics. These fascinating states, however, usually are located at the interfaces or surfaces of ultrathin systems due to the short interaction range of the Dzyaloshinskii–Moriya interaction (DMI). Here, magnetic topological states in a 40-unit cells (16 nm) SrRuO₃ layer are successfully created via an interlayer exchange coupling mechanism and the interfacial DMI. By controlling the thickness of an antiferromagnetic and ferromagnetic layer, interfacial ionic polarization, as well as the transformation between ferromagnetic and magnetic topological states, can be modulated. Using micromagnetic simulations, the formation and stability of robust magnetic skyrmions in SrRuO₃/BiFeO₃ heterostructures are elucidated. Magnetic skyrmions in thick multiferroic heterostructures are promising for the development of topological electronics as well as rendering a practical approach to extend the interfacial topological phenomena to bulk via antiferromagnetic order.     

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO₃, graphene/LaAlO₃/SrTiO₃, graphene/ferroelectric oxide, MoS₂/SrTiO₃, and FeSe/SrTiO₃ heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next‐generation optoelectronic devices.     

Crystallization Kinetics of Cubic Boron Nitride in the B-N-Mg-H-F System

The kinetics of the h-BN → c-BN transformation in a mixture of composition 78% h-BN + 5% NH₄F + 2% B + 15% Mg are studied at 5 GPa and temperatures from 1670 to 1910 K. The activation energy of c-BN formation, temperature-dependent reaction rate constant, strength of the forming c-BN particles, and density of paramagnetic defects in c-BN are evaluated.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 816–818.Original Russian Text Copyright © 2005 by Shipilo, Ignatenko, Anichenko, Azarko.     

A Monolayer MoS2 FET with an EOT of 1.1 nm Achieved by the Direct Formation of a High-κ Er2O3 Insulator Through Thermal Evaporation

Achieving the direct growth of an ultrathin gate insulator with high uniformity and high quality on monolayer transition metal dichalcogenides (TMDCs) remains a challenge due to the chemically inert surface of TMDCs. Although the main solution for this challenge is utilizing buffer layers before oxide is deposited on the atomic layer, this method drastically degrades the total capacitance of the gate stack. In this work, we constructed a novel direct high-κ Er₂O₃ deposition system based on thermal evaporation in a differential-pressure-type chamber. A uniform Er₂O₃ layer with an equivalent oxide thickness of 1.1 nm was achieved as the gate insulator for top-gated MoS₂ field-effect transistors (FETs). The top gate Er₂O₃ insulator without the buffer layer on MoS₂ exhibited a high dielectric constant that reached 18.0, which is comparable to that of bulk Er₂O₃ and is the highest among thin insulators (< 10 nm) on TMDCs to date. Furthermore, the Er₂O₃/MoS₂ interface (D_it ≈ 6 × 10¹¹ cm⁻² eV⁻¹) is confirmed to be clean and is comparable with that of the h-BN/MoS₂ heterostructure. These results prove that high-quality dielectric properties with retained interface quality can be achieved by this novel deposition technique, facilitating the future development of 2D electronics.     

Bio-inspired structured boron carbide-boron nitride composite by reactive spark plasma sintering

Nature creates composite materials with complex hierarchical structures that possess impressive mechanical properties enhancement capabilities. An approach to improve mechanical properties of conventional composites is to mimic the biological material structured ‘hard’ core and ‘soft’ matrix system. This would allow the efficient transfer of load stress, dissipation of energy and resistance to cracking in the composite. In the current study, reactive spark plasma sintering (SPS) of boron carbide B₄C was carried out in a nitrogen N₂ gas environment. The process created a unique core-shell structured material with the potential to form a high impact-resistant composite. Transmission electron microscopy observation of nitrided-B₄C revealed the encapsulation of B₄C grains by nano-layers of hexagonal-boron nitride (h-BN). The effect of the h-BN contents on hardness were measured using micro- and nano-indentation. Commercially available h-BN was also mechanically mixed and sintered with B₄C to compare the effectiveness of nitrided B₄C. Results have shown that nitrided B₄C has a higher hardness value and the optimum content of h-BN from nitridation was 0.4%wt with the highest nano-indentation hardness of 56.7 GPa. The high hardness was attributed to the h-BN matrix situated between the B₄C grain boundaries which provided a transitional region for effective redistribution of the stress in the material.     

Electric and magnetic properties of ferromagnetic/piezoelectric bilayered composite

One of the most promising ways for the realization of multi-functional materials is the integration of oxides with different properties in artificial heterostructures. In this paper, a novel piezoelectric–ferromagnetic heterostructure consisting of 0.92Na_0.5Bi_0.5TiO₃–0.08BaTiO₃ (abbreviated as BNT–BT_0.08) and CoFe₂O₄ layers is fabricated on Si–Pt substrate, by sol–gel method coupled with spin-coating technique. The composite thin film shows only perovskite Bi_0.5Na_0.5TiO₃-like rhombohedral phase and CoFe₂O₄ cubic phase. The thickness of CoFe₂O₄ and BNT–BT_0.08 layers is ~?280 and?~?400 nm, respectively. BNT–BT_0.08/CoFe₂O₄ heterostructure thin film shows a saturation magnetization of 0.11 emu/g at 5 K and 0.07 emu/g at 295 K, dielectric constant of 235 at 1 kHz and tunability of 70% at 1 kHz and an electric field E?=?110 kV/cm. The results reveal that the investigated hybrid piezoelectric/ferromagnetic structure shows piezoelectric behavior, good ferroelectric and ferromagnetic properties. This bilayer composite can be used in miniature low-frequency magnetic sensor and piezoelectric sensor for biomedical domain.