Jiuxiang Dai

Nature Communications, Published online: 16 May 2024; doi:10.1038/s41467-024-48522-8

Rhombohedral-stacked (R-stacked) transition metal dichalcogenide bilayers exhibit remarkable properties, but their large-area epitaxial growth remains challenging. Here, the authors report the remote epitaxy of centimetre-scale single-crystal R-stacked WS2 bilayer films on sapphire substrates.

2D ferroelectric metalens, composed of CuInP₂S₆, is demonstrated with electrically tunability. The linear electric-field modulation of light is verified in 2D ferroelectric CuInP₂S₆, yielding an effective electro-optic coefficient r _c of 20.28 pm V^–1. The 2D ferroelectric Fresnel metalens shows efficacious focusing ability with an electrical modulation efficiency of the focusing exceeding 34%.

Abstract

The advent of 2D ferroelectrics, characterized by their spontaneous polarization states in layer-by-layer domains without the limitation of a finite size effect, brings enormous promise for applications in integrated optoelectronic devices. Comparing with semiconductor/insulator devices, ferroelectric devices show natural advantages such as non-volatility, low energy consumption and high response speed. Several 2D ferroelectric materials have been reported, however, the device implementation particularly for optoelectronic application remains largely hypothetical. Here, the linear electro-optic effect in 2D ferroelectrics is discovered and electrically tunable 2D ferroelectric metalens is demonstrated. The linear electric-field modulation of light is verified in 2D ferroelectric CuInP₂S₆. The in-plane phase retardation can be continuously tuned by a transverse DC electric field, yielding an effective electro-optic coefficient r _c of 20.28 pm V^–1. The CuInP₂S₆ crystal exhibits birefringence with the fast axis oriented along its (010) plane. The 2D ferroelectric Fresnel metalens shows efficacious focusing ability with an electrical modulation efficiency of the focusing exceeding 34%. The theoretical analysis uncovers the origin of the birefringence and unveil its ultralow light absorption across a wide wavelength range in this non-excitonic system. The van der Waals ferroelectrics enable room-temperature electrical modulation of light and offer the freedom of heterogeneous integration with silicon and another material system for highly compact and tunable photonics and metaoptics.

npj 2D Materials and Applications, Published online: 16 May 2024; doi:10.1038/s41699-024-00473-w

Controlled epitaxy and patterned growth of one-dimensional crystals via surface treatment of two-dimensional templates

Nature Reviews Chemistry, Published online: 16 May 2024; doi:10.1038/s41570-024-00605-2

Intercalation of atoms, ions and molecules is a powerful tool for finely regulating atomically thin, 2D materials. This Review highlights the effects of intercalation in 2D materials and discusses their in situ studies.

This review primarily focuses on elucidating the factors that influence their interfacial growth, with a specific emphasis on the interfacial engineering of heterogeneous reactions, in which MOF-on-MOF heterostructures can be conveniently obtained by using pre-fabricated MOF precursors. These factors are categorized as internal and external elements, including inorganic metals, organic ligands, lattice matching, nucleation kinetics, thermodynamics, etc.

Abstract

Metal–organic frameworks (MOFs), as a subclass of porous crystalline materials with unique structures and multifunctional properties, play a pivotal role in various research domains. In recent years, significant attention has been directed toward composite materials based on MOFs, particularly MOF-on-MOF heterostructures. Compared to individual MOF materials, MOF-on-MOF structures harness the distinctive attributes of two or more different MOFs, enabling synergistic effects and allowing for the tailored design of diverse multilayered architectures to expand their application scope. However, the rational design and facile synthesis of MOF-on-MOF composite materials are in principle challenging due to the structural diversity and the intricate interfaces. Hence, this review primarily focuses on elucidating the factors that influence their interfacial growth, with a specific emphasis on the interfacial engineering of heterogeneous reactions, in which MOF-on-MOF hybrids can be conveniently obtained by using pre-fabricated MOF precursors. These factors are categorized as internal and external elements, encompassing inorganic metals, organic ligands, lattice matching, nucleation kinetics, thermodynamics, etc. Meanwhile, these intriguing MOF-on-MOF materials offer a wide range of advantages in various application fields, such as adsorption, separation, catalysis, and energy-related applications. Finally, this review highlights current complexities and challenges while providing a forward-looking perspective on future research directions.

Complex multimaterial lateral heterostructures of van der Waals semiconductors are fabricated by sequential growth of different multilayer Ge and Sn monochalcogenides. Such designer materials with judiciously placed interfaces provide access to unique properties, exemplified here by the selection of photonic modes with discrete energies in a laterally embedded active layer through Fabry–Perot interference rather than conventional band engineering.

Abstract

Lateral heterostructures combining two multilayer group IV chalcogenide van der Waals semiconductors have attracted interest for optoelectronics, twistronics, and valleytronics, owing to their structural anisotropy, bulk-like electronic properties, enhanced optical thickness, and vertical interfaces enabling in-plane charge manipulation/separation, perpendicular to the trajectory of incident light. Group IV monochalcogenides support propagating photonic waveguide modes, but their interference gives rise to complex light emission patterns throughout the visible/near-infrared range both in uniform flakes and single-interface lateral heterostructures. Here, this work demonstrates the judicious integration of pure and alloyed monochalcogenide crystals into multimaterial heterostructures with unique photonic properties, notably the ability to select photonic modes with targeted discrete energies through geometric factors rather than band engineering. SnS-GeS_1−x Se _x -GeSe-GeS_1−x Se _x heterostructures with a GeS_1−x Se_x active layer sandwiched laterally between GeSe and SnS, semiconductors with similar optical constants but smaller bandgaps, were designed and realized via sequential vapor transport synthesis. Raman spectroscopy, electron microscopy/diffraction, and energy-dispersive X-ray spectroscopy confirm a high crystal quality of the laterally stitched components with sharp interfaces. Nanometer-scale cathodoluminescence spectroscopy provides evidence for a facile transfer of electron–hole pairs across the lateral interfaces and demonstrates the selection of photon emission at discrete energies in the laterally embedded active (GeS_{1− x} Se _x ) part of the heterostructure.

By utilizing a bottom-up approach, meticulous regulation over the epitaxial growth of h-CoO and ZnO on one another is effectively attained, facilitating the fabrication of 3D stereographic structures with customizable geometries. The resulting hybrid nanocrystals exhibit enhanced catalytic performance when contrasted with single-component nanomaterials, primarily due to their distinctive structural characteristics.

Abstract

The rational design of hybrid nanocrystals structures facilitates electronic and energetic communication between different component, which can optimize their specific performance. In this study, an efficient approach for building intricate ZnO@h-CoO nanocomposites and their derivatives is presented, based on a lattice-match/mismatch mechanism. Due to the ultra-low lattice mismatch between ZnO and hexagonal CoO (as low as 0.18%), the h-CoO layer enables epitaxial growth on the ZnO templates, and ZnO can also grow epitaxially outside the CoO layer with ease. Similarly, the thickness of the epitaxial layer and the number of alternating layers can be adjusted arbitrarily. In contrast to h-CoO, the growth of cubic crystalline oxides (such as MnO) on ZnO results in the formation of nanoparticles due to a large mismatch index (following the Volmer–Weber models). Interestingly, when h-CoO is introduced as a further component into the MnO/ZnO composite, the cubic crystalline particles on the surface of the ZnO do not disturb the epitaxial growth of the h-CoO, allowing for the formation of nanocomposites with more components. Furthermore, additional units can be added to the nanocomposite further based on the lattice-match/mismatch mechanism, which is analogous to the building nano-bricks.

A family of conductive van der Waals solid, transition metal carbo-chalcogenide is screened and predicted to be an excellent Li⁺/Na⁺ host, storing near double-layer lithium/sodium and maintaining the structure and voltage, combining with open ion migration paths, holds promises as novel rechargeable battery materials.

Abstract

High-rate lithium/sodium ion batteries or capacitors are the most promising functional units to achieve fast energy storage that highly depends on charge host materials. Host materials with lamellar structures are a good choice for hybrid charge storage hosts (capacitor or redox type). Emerging layered transition metal carbo-chalcogenides (TMCC) with homogeneous sulfur termination are especially attractive for charge storage. Using density functional theory calculations, six of 30 potential TMCC are screened to be stable, metallic, anisotropic in electronic conduction and mechanical properties due to the lamellar structures. Raman, infrared active modes and frequencies of the six TMCC are well assigned. Interlayer coupling, especially binding energies predict that the bulk layered materials can be easily exfoliated into 2D monolayers. Moreover, Ti₂S₂C, Zr₂S₂C are identified as the most gifted Li⁺/Na⁺ anode materials with relatively high capacities, moderate volume expansion, relatively low Li⁺/Na⁺ migration barriers for batteries or ion-hybrid capacitors. This work provides a foundation for rational materials design, synthesis, and identification of the emerging 2D family of TMCC.

This article presents the distinct thickness-related properties of boron nitride nanosheets (BNNSs), encompassing Raman signatures, unique adsorption behavior, mechanical properties, thermal conductivity, and thermal expansion coefficients. It delves into the mechanisms governing thickness effects and explores BNNS applications in surface-enhanced Raman spectroscopy, metal-enhanced fluorescence, and thermal management.

Abstract

Owing to its exceptional properties and wide-ranging potential applications from aerospace to medicine, hexagonal boron nitride (h-BN) has garnered considerable attention over the past decades. Boron nitride nanosheets (BNNSs), atomically thin h-BN, not only inherit most of the outstanding properties of h-BN but also exhibit superior characteristics compared to their bulk counterpart due to their reduced thickness, such as special adsorption behaviors and enhanced thermal conductivity. Furthermore, BNNSs display distinct thickness-dependent properties from graphene and other 2D materials, such as unique mechanical response under indentation. This feature article provides an overview of the thickness-related special properties of BNNSs, primarily derived from mechanically exfoliated h-BN single crystals. These properties span various domains, including Raman signatures, molecule adsorption-induced conformational changes, mechanical properties, thermal conductivity, and thermal expansion coefficients. Moreover, the feature article explores the underlying mechanisms governing these atomic-scale thickness effects. Leveraging their unique properties, the feature article investigates diverse applications of BNNSs, encompassing surface-enhanced Raman spectroscopy, metal-enhanced fluorescence, and isotropic thermal management.

Nature Nanotechnology, Published online: 15 May 2024; doi:10.1038/s41565-024-01675-5

Superionic fluoride dielectrics with a low ion migration barrier are capable of excellent capacitive coupling and are highly compatible with scalable device manufacturing processes for integrated electronics.

Large-area continuous thin films of metastable trigonal Cr₂S₃ are grown by ultralow gas-flow governed dynamic transport methods. The anisotropic SHG response of as-prepared Cr₂S₃ demonstrated effective second-order nonlinearity of 48.0 pm V⁻¹, in which sulfur vacancies and dangling bonds could break the surface central-symmetry and contribute nonlinear optical polarizabilities, providing a new understanding of SHG for nonlayered 2D materials.

Abstract

As for nonlayered 2D polymorphic materials, especially for Cr-based chalcogenides, large-area thin film growth with phase control is considered the most important synthesis challenge for magnetic, electronic, and optoelectronic devices. However, the synthesis methods of large continuous thin films for nonlayered 2D materials are still limited and rarely reported, also for the phase control growth, which is inhibited by isotropic 3D growth and similar Gibbs free energy for different phases. Herein, enhanced mass transport chemical vapor deposition is established to achieve the control synthesis of trigonal Cr₂S₃ thin films, in which the stable boundary layer supplies the continuous reaction species and tunes the reaction kinetics. The trigonal phase formation is confirmed by atomic structure characterization, optical absorption and piezoelectric measurements, demonstrating unique physical properties different from rhombohedral phase. The trigonal Cr₂S₃ thin films show obvious layer independent and dissimilar angle-resolved harmonic generation, indicating the surface broken symmetry that can be understood by the combination of negligible piezoelectric response for bulk. The work presents the large-area synthesized strategy by the modification of mass transport for nonlayered 2D materials with new phase formation and establishes the surface symmetry breaking dominated SHG mechanism for future nonlinear optical materials.

2D polarization materials are promising for miniaturized devices due to their unique properties. However, strategies for creating 2D polarization through new mechanisms are rare. This work introduces a 2D Janus structure with both vertical and planar polarization, achieved by applying force with a nanoprobe. This technique breaks symmetry at the atomic level, inducing polarization and paving the way for new 2D polarization materials' development and design.

Abstract

2D polarization materials have emerged as promising candidates for meeting the demands of device miniaturization, attributed to their unique electronic configurations and transport characteristics. Although the existing inherent and sliding mechanisms are increasingly investigated in recent years, strategies for inducing 2D polarization with innovative mechanisms remain rare. This study introduces a novel 2D Janus state by modulating the puckered structure. Combining scanning probe microscopy, transmission electron microscopy, and density functional theory calculations, this work realizes force-triggered out-of-plane and in-plane dipoles with distorted smaller warping in GeSe. The Janus state is preserved after removing the external mechanical perturbation, which could be switched by modulating the sliding direction. This work offers a versatile method to break the space inversion symmetry in a 2D system to trigger polarization in the atomic scale, which may open an innovative insight into configuring novel 2D polarization materials.

The orthogonality of the electrochemical trigger and the optical readout distinguishes electrochemiluminescence (ECL) from classic microscopy and electrochemical techniques. In this review, we summarize the recent advances in ECL microscopy, emphasizing original configurations which enable the mapping of the (electro)chemical reactivity and the imaging of biological entities. Finally, we highlight the recent achievements in imaging single ECL photons or molecules.

Abstract

Electrochemiluminescence (ECL) is rapidly evolving from an analytical method into an optical microscopy. The orthogonality of the electrochemical trigger and the optical readout distinguishes it from classic microscopy and electrochemical techniques, owing to its near-zero background, remarkable sensitivity, and absence of photobleaching and phototoxicity. In this minireview, we summarize the recent advances in ECL imaging technology, emphasizing original configurations which enable the imaging of biological entities and the improvement of the analytical properties by increasing the complexity and multiplexing of bioassays. Additionally, mapping the (electro)chemical reactivity in space provides valuable information on nanomaterials and facilitates deciphering ECL mechanisms for improving their performances in diagnostics and (electro)catalysis. Finally, we highlight the recent achievements in imaging at the ultimate limits of single molecules, single photons or single chemical reactions, and the current challenges to translate the ECL imaging advances to other fields such as material science, catalysis and biology.

This work reviews the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices are reviewed followed by the integration of perception, memory, and computing application. Notably, the 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing.

Abstract

The quantity of sensor nodes within current computing systems is rapidly increasing in tandem with the sensing data. The presence of a bottleneck in data transmission between the sensors, computing, and memory units obstructs the system's efficiency and speed. To minimize the latency of data transmission between units, novel in-memory and in-sensor computing architectures are proposed as alternatives to the conventional von Neumann architecture, aiming for data-intensive sensing and computing applications. The integration of 2D materials and 2D ferroelectric materials has been expected to build these novel sensing and computing architectures due to the dangling-bond-free surface, ultra-fast polarization flipping, and ultra-low power consumption of the 2D ferroelectrics. Here, the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing is reviewed. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices, are reviewed followed by the integration of perception, memory, and computing application. Notably, 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing. As an emerging device configuration, 2D ferroelectric devices have the potential to expand into the sensor-memory and computing integration application field, leading to new possibilities for modern electronics.

Herein, by using polyacrylonitrile as the transfer medium and encapsulating layer, wafer-scale graphene transfer, devices fabrication, and efficient encapsulation against the airborne contaminations are successfully achieved, and the transferred graphene delivers improved electronic performances and long-term stability, offering a reliable method for fabricating wafer-scale devices of 2D materials with outstanding electronic quality.

Abstract

The disparity between growth substrates and application-specific substrates can be mediated by reliable graphene transfer, the lack of which currently strongly hinders the graphene applications. Conventionally, the removal of soft polymers, that support the graphene during the transfer, would contaminate graphene surface, produce cracks, and leave unprotected graphene surface sensitive to airborne contaminations. In this work, it is found that polyacrylonitrile (PAN) can function as polymer medium for transferring wafer-size graphene, and encapsulating layer to deliver high-performance graphene devices. Therefore, PAN, that is compatible with device fabrication, does not need to be removed for subsequent applications. The crack-free transfer of 4 in. graphene onto SiO₂/Si wafers, and the wafer-scale fabrication of graphene-based field-effect transistor arrays with no observed clear doping, uniformly high carrier mobility (≈11 000 cm² V⁻¹ s⁻¹), and long-term stability at room temperature, are achieved. This work presents new concept for designing the transfer process of 2D materials, in which multifunctional polymer can be retained, and offers a reliable method for fabricating wafer-scale devices of 2D materials with outstanding performance.

This review provides an overview of hot-carrier engineered two-dimensional (2D) infrared optoelectronic devices. Basic principles of hot-carrier dynamics and photoelectric conversion are clarified in detail. The progresses of 2D infrared hot-carrier optoelectronic devices are summarized, with a specific emphasis on photodetectors, solar cells, light-emitting devices and novel functionalities. The challenges and prospects of hot-carrier device towards infrared applications are highlighted.

Abstract

Plasmonic hot carrier engineering holds great promise for advanced infrared optoelectronic devices. The process of hot carrier transfer has the potential to surpass the spectral limitations of semiconductors, enabling detection of sub-bandgap infrared photons. By harvesting hot carriers prior to thermalization, energy dissipation is minimized, leading to highly efficient photoelectric conversion. Distinguished from conventional band-edge carriers, the ultrafast interfacial transfer and ballistic transport of hot carriers present unprecedented opportunities for high-speed photoelectric conversion. However, a complete description on the underlying mechanism of hot-carrier infrared optoelectronic device is still lacking, and the utilization of this strategy for tailoring infrared response is in its early stages. This review aims to provide a comprehensive overview of the generation, transfer and transport dynamics of hot carriers. Basic principles of hot-carrier conversion in heterostructures are discussed in detail. In addition, progresses of two-dimensional (2D) infrared hot-carrier optoelectronic devices are summarized, with a specific emphasis on photodetectors, solar cells, light-emitting devices and novel functionalities through hot-carrier engineering. Furthermore, challenges and prospects of hot-carrier device towards infrared applications are highlighted.

Nature Materials, Published online: 13 May 2024; doi:10.1038/s41563-024-01894-0

Ultrathin and flat crystals of bismuth are grown between the atomically flat layers of a van der Waals material. These crystals exhibit outstanding electronic properties, including gate-tunable quantum oscillations of the magnetoresistance.

Ferroelectric polarization can be formed in some non-polar point groups when the edges break the crystal symmetry. Such polarizations can be maintained at macroscale and are switchable if the transition between multiple equivalent states with high symmetry can be realized via long ion displacements.

Abstract

In the classical model, ferroelectricity is associated with small ion displacements from paraelectric phases with high symmetry, and ferroelectric crystals must adopt one of the ten polar point groups with low symmetry according to Neumann's principle. In this work, it is proposed that this conclusion is based on perfect bulk crystals without taking the boundaries into account. First-principles evidence shows that ferroelectric polarizations may also be formed in some non-polar point groups as the edges generally break the crystal symmetry. Meanwhile, such polarizations can be maintained at macroscale and are switchable when the transition between multiple equivalent states with high symmetry can be realized via long ion displacements， essentially akin to ion conductors. For example, the switching barriers can be much reduced in sliding ferroelectric bilayer systems or ionic compounds with covalent-like directionality. Such unconventional ferroelectricity can be attributed to the boundaries and long ion displacements, and its existence in several systems like CuCrS₂ is supported by experimental observations. It may explain a series of unclarified phenomena reported previously as well as significantly expand the scope of ferroelectrics, especially those with high polarizations induced by long ion displacements.

A polarization-sensitive GeS₂/PbSe two-dimensional (2-D short-wave infrared (SWIR) spectral phototransistor, integrated with an artificial neural network (ANN) for deep-learning, reconstructs infrared spectra (900–1700 nm) with tunable V_G and δ. This detector achieves 96.7% accuracy in narrow-band selection, supports at least 8 multi-wavelengths’ channels, and narrow FWHM (15 nm), which is crucial for future hyperspectral detecting applications.

Abstract

Integrated computational spectrometers with gate-tunable nano heterostructures and reconstruction algorithms are attractive for on-chip gas-sensing spectrometers and have enabled versatile spectrum detectors. However, they require the selective and optical filtering capabilities of wavelengths, restricting their efficient implementation in narrow-band photodetection. In this study, a printable spectral phototransistor is developed with high dynamic detectivity (10¹² Jones and 10⁵ Hz at −3 db bandwidth) modulated by a GeS₂ nanosheet heterostructure at short-wave infrared (SWIR) regime. Using the transport mode switching of carriers in a heterostructure and the polarization-sensitivity of the GeS₂ two-dimension (2-D) nanosheet, this SWIR spectral phototransistor demonstrates an accurate narrow-band selective (96.7% accuracy) spectrum detector and performed a deep-learning analysis of an artificial neural network (ANN). Furthermore, this GeS₂ 2-D based spectral phototransistor, characterized by its high in-plane anisotropy and electrically reconfigurable properties, extends the applicability of narrow-band photodetection with 15 nm Full Width at Half Maximum (FWHM) to the recognition of trace-gases at the parts per billion (ppb) level.

Nature Communications, Published online: 14 May 2024; doi:10.1038/s41467-024-48431-w

Antiferromagnetic spintronics offer high speed operations, and reduced issues with stray fields compared to ferromagnetic systems, however, antiferromagnets are typically more challenging to manipulate electrically. Here, Yang, Kim, and coauthors demonstrate electrical control of magnon dispersion and frequency in an α-Fe2O3/Pt heterostructure.

Nature Photonics, Published online: 14 May 2024; doi:10.1038/s41566-024-01434-x

Exploiting the interactions between bright excitons and free carriers in an atomically thin semiconductor of trilayer tungsten diselenide WSe2 results in Fermi polarons that exhibit unusually large nonlinearity.

Nature Communications, Published online: 13 May 2024; doi:10.1038/s41467-024-48429-4

Here, the authors demonstrate a wafer-scale, low-temperature process using atomic layer deposition, for the synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. They further fabricate aBN-encapsulated monolayer MoS2 field-effect transistors.