Jiuxiang Dai

The in situ growth of wafer-scale graphene patterns on diverse substrates is proposed by using photosensitive polymer as a solid carbon source. It enables graphene/n-AlGaN heterojunction for the fabrication of optoelectronic artificial synaptic device array, which can mimic key functionalities of biological synapses, also revealing the visual learning ability like that of a human brain.

Abstract

The unique optical and electrical properties of graphene-based heterojunctions make them significant for artificial synaptic devices, promoting the advancement of biomimetic vision systems. However, mass production and integration of device arrays are necessary for visual imaging, which is still challenging due to the difficulty in direct growth of wafer-scale graphene patterns. Here, a novel strategy is proposed using photosensitive polymer as a solid carbon source for in situ growth of patterned graphene on diverse substrates. The growth mechanism during high-temperature annealing is elucidated, leading to wafer-scale graphene patterns with exceptional uniformity, ideal crystalline quality, and precise control over layer number by eliminating the release of volatile from oxygen-containing resin. The growth strategy enables the fabrication of two-inch optoelectronic artificial synaptic device array based on graphene/n-AlGaN heterojunction, which emulates key functionalities of biological synapses, including short-term plasticity, long-term plasticity, and spike-rate-dependent plasticity. Moreover, the mimicry of visual learning in the human brain is attributed to the regulation of excitatory and inhibitory post-synapse currents, following a learning rule that prioritizes initial recognition before memory formation. The duration of long-term memory reaches 10 min. The in situ growth strategy for patterned graphene represents the novelty for fabricating fundamental hardware of an artificial neuromorphic system.

The method of salt-assisted vapor–liquid–solid (VLS) growth is introduced to synthesize 1D nanostructures of trichalcogenide van der Waals (vdW) materials, exemplified by niobium trisulfide (NbS₃). The method uses a unique catalyst consisting of an alloy of Au and an alkali metal halide (NaCl) to enable rapid and directional growth.

Abstract

The method of salt-assisted vapor–liquid–solid (VLS) growth is introduced to synthesize 1D nanostructures of trichalcogenide van der Waals (vdW) materials, exemplified by niobium trisulfide (NbS₃). The method uses a unique catalyst consisting of an alloy of Au and an alkali metal halide (NaCl) to enable rapid and directional growth. High yields of two types of NbS₃ 1D nanostructures, nanowires and nanoribbons, each with sub-ten nanometer diameter, tens of micrometers length, and distinct 1D morphology and growth orientation are demonstrated. Strategies to control the location, size, and morphology of growth, and extend the growth method to synthesize other transition metal trichalcogenides, NbSe₃ and TiS₃, as nanowires are demonstrated. Finally, the role of the Au–NaCl alloy catalyst in guiding VLS synthesis is described and the growth mechanism based on the relationships measured between structure (growth orientation, morphology, and dimensions) and growth conditions (catalyst volume and growth time) is discussed. These results introduce opportunities to expand the library of emerging 1D vdW materials to make use of their unique properties through controlled growth at nanoscale dimensions.

Non-volatile floating-gate memory devices with all functional layers made of 2D materials with atomically sharp interface, including MoS₂ channel layer, hBN tunnel layer, multilayer graphene (MLG) floating-gate layer, hBN block layer and MLG control gate layer, can be ultrafastly programmed/erased in ≈20 ns with high extinction ratios (up to 10⁸) and long-term retention characteristics.

Abstract

The explosive growth of massive-data storage and the demand for ultrafast data processing require innovative memory devices with exceptional performance. 2D materials and their van der Waal heterostructures with atomically sharp interfaces hold great promise for innovations in memory devices. Here, this work presents non-volatile, floating-gate memory devices with all functional layers made of 2D materials, achieving ultrafast programming/erasing speeds (20 ns), high extinction ratios (up to 10⁸), and multi-bit storage capability. These devices also exhibit long-term data retention exceeding 10 years, facilitated by a high gate-coupling ratio (GCR) and atomically sharp interfaces between functional layers. Additionally, this work demonstrates the realization of an “OR” logic gate on a single-device unit by synergistic electrical and optical operations. The present results provide a solid foundation for next-generation ultrahigh-speed, ultralong lifespan, non-volatile memory devices, with a potential for scale-up manufacturing and flexible electronics applications.

Bidirectionally high-thermally conductive graphene thick film is achieved by a reliable seamless bonding assembly strategy. The graphene thick film with 250 µm demonstrates record κ_∥ of 925.75 W (mK)⁻¹ and κ_⊥ of 7.03 W (mK)⁻¹, meanwhile exhibiting remarkable stability even after hundreds of cycled harsh temperature shocks from 77 to 573 K, ensuring its environmental adaptability for extreme thermal management.

Abstract

With the rapid development of high-power electronics in aerospace, communication, and energy storage systems, the huge heat flux poses an increasing threat to the safety of electronic devices. Compared with thin films of a few micro thicknesses, high-quality graphene thick film (GTF) exceeding hundreds of microns thickness is a promising candidate to solve thermal management challenges owing to higher heat-flux. However, traditional GTF usually has lower thermal conductivity and weak mechanical properties attributed to disordered sheet alignment and frail interfacial adhesion. Here, a seamless bonding assembly (SBA) strategy is proposed to attain GTF over record hundreds of microns with robust coalescence interfaces. For the GTF-SBA with ≈250 µm thickness, the in-plane and through-plane thermal conductivities are 925.75 and 7.03 W m⁻¹ K⁻¹, approximately two times and 12 times those of the GTF prepared by traditional adhesive assembly method, respectively. Furthermore, the GTF-SBA demonstrates remarkable stability even after cycled harsh temperature shocks from 77 to 573 K, ensuring its environmental adaptability for long-term service in extreme conditions. These findings provide valuable insights into the interfacial design of graphene bulk materials and highlight the potential applications of high-performance graphene-based materials for extreme thermal management demands.

The approach involves in situ 2D material growth on Si/SiO₂ substrates facilitated by the CVD system, followed by the gate-last processing integration of devices. This demonstrates seamless compatibility with CMOS technologies, based on wafer-scale monolayer 2D semiconductors, showcasing potential in transistor and encapsulated photodetector applications. This research paves the way for moving 2D semiconductor manufacturing from laboratory to industrial scale.

Abstract

2D semiconductors have emerged as candidates for next-generation electronics. However, previously reported 2D transistors which typically employ the gate-first process to fabricate a back-gate (BG) configuration while neglecting the thorough impact on the dielectric capping layer, are severely constrained in large-scale manufacturing and compatibility with complementary metal–oxide–semiconductor (CMOS) technology. In this study, dual-gate (DG) field-effect transistors have been realized based on wafer-scale monolayer MoS₂ and the gate-last processing, which avoids the transfer process and utilizes an optimized top-gate (TG) dielectric stack, rendering it highly compatible with CMOS technology. Subsequently, the physical mechanism of TG dielectric deposition and the corresponding controllable threshold voltage (V _TH) shift is investigated. Then the fabricated TG-devices with a large on/off ratio up to 1.7 × 10⁹, negligible hysteresis (≈14 mV), and favorable stability. Additionally, encapsulated TG structured photodetectors have been demonstrated which exhibit photo responsivity (R) up to 9.39 × 10³ A W⁻¹ and detectivity (D ^*) ≈2.13 × 10¹³ Jones. The result paves the way for future CMOS-compatible integration of 2D semiconductors for complex multifunctional IC applications.

Nature, Published online: 20 March 2024; doi:10.1038/s41586-024-07214-5

Publication date: August 2024

Source: Progress in Materials Science, Volume 144

Author(s): Kassa Belay Ibrahim, Tofik Ahmed Shifa, Sandro Zorzi, Marshet Getaye Sendeku, Elisa Moretti, Alberto Vomiero

The strong non-covalent attachment of electrolytic species onto the 2D materials’ basal plane alters their interlayer interactions, allowing for scrolling and self-assembly into sturdy fibers without disrupting the pristine crystalline structure.

Abstract

2D materials are solid microscopic flakes with a-few-Angstrom thickness possessing some of the largest surface-to-volume ratios known. Altering their conformation state from a flat flake to a scroll or fiber offers a synergistic association of properties arising from 2D and 1D nanomaterials. However, a combination of the long-range electrostatic and short-range solvation forces produces an interlayer repulsion that has to be overcome, making scrolling 2D materials without disrupting the pristine structure a challenging task. Herein, a facile method is presented to alter the 2D materials’ inter-layer interactions by confining organic salts onto their basal area, forming 2D-confined electrolytes. The confined electrolytes produce local charge inhomogeneities, which can conjugate across the interlayer gap, binding the two surfaces. This allows the 2D-confined electrolytes to behave as polyelectrolytes within a higher dimensional order (2D → 1D) and form robust nanofibers with distinct electronic properties. The method is not material-specific and the resulting fibers are tightly bound even though the crystal structure of the basal plane remains unaltered.

This study demonstrates a direct, single-step, and ultrafast protocol i.e., plasma spraying technique that simultaneously exfoliates and transforms bulk TMDs (MoS₂ and WS₂) having 2H-phase into an ultrathin-layers with an absolute (i.e., 100%) 1T-phase at a large scale without the usage of any intercalates or solvents. These synthesized high-quality ultrathin layers of 1T-TMDs can be commercially exploited in energy storage devices.

Abstract

Large-scale production of high-quality ultrathin layers (1–3 nm) of molybdenum disulfide (MoS₂) with absolute (≈100%) 1T-phase is still in its infancy. Therefore, it is extremely crucial to have a technique for the mass production of ultrathin 1T-MoS₂ layers. Here, a direct, single-step, and ultra-fast technique that produces high-quality ultrathin layers of 1T-MoS₂ with a production rate as high as 58 g h⁻¹ without the usage of any intercalates or solvents is demonstrated. The exfoliated ultrathin 1T-MoS₂ layers exhibited ≈100% 1T-phase with a large specific surface area (67 m² g⁻¹), higher electrical conductivity (140 S m⁻¹), high thermal stability (up to 500 °C) and hydrophilicity (water contact angle (WCA): ≈23.4⁰). The ultrathin 1T-MoS₂ layers showed a higher specific capacitance of 420 F g⁻¹; perhaps an ideal candidate for the electrodes of supercapacitors. Moreover, the ultrathin 1T-MoS₂ layer exhibited better mechanical flexibility and retained its original performance on bending between 0 and 180⁰ angles. Further, to assess the adeptness of the protocol, An initial trial is done on other transition metal dichalcogenides (TMDs) i.e., tungsten disulfide (WS₂), and observe similar results. The work sheds light on the simultaneous exfoliation and phase transformation of TMDs in large quantities, and detailed proofs-of-concept demonstrate its application in next-generation energy storage devices.

Four classes of tessellations composed of chains of non-hexagonal rings separated by nanoribbons of hexagonal rings are established to construct highly stable 2D crystals. Based on the interaction between polygonal rings, a modified Read–Shockley model is further proposed to describe the stability of such 2D crystals, and peculiar topological electronic structures are found.

Abstract

Reticular chemistry has been a cornerstone in the design of novel 2D materials. Despite numerous possibilities for topological arrangements, only a few with high symmetry can form stable networks. Here, starting from 2D carbons, four types of highly stable tessellations are discovered, which consist of chains of non-hexagonal rings separated by hexagonal ribbons. A modified Read–Shockley model is established to perfectly describe the stability of these highly stable frameworks, which is based on the interaction between non-hexagonal rings. Moreover, these four types of tessellations and the modified Read–Shockley model are found to be of general validity in designing highly stable 2D materials, which is verified by the calculations on polymorphs of boron nitride and molybdenum disulfide. Besides, among the studied 2D carbon allotropes, two semi-metallic structures with highly anisotropic Dirac cones and one semimetal with a Dirac nodal line at the Fermi level are discovered, as protected by their D _2h symmetry. Spin-orbital coupling is further found to open small bandgaps for these three Dirac structures, making them nontrivial topological insulators. The in-depth understanding of the stability of 2D crystals in this study provides a new way for rational design of 2D crystals that may show peculiar electronic structures.

A self-driven photodetector with high polarization sensitivity is realized by a broken-gap ReSe₂/SnSe₂ vdWH with semi-vertical geometry. The multilayer graphene/ReSe₂/SnSe₂ device exhibits excellent potential applications in polarized imaging and polarization-coded optical communication with quaternary logic state. This work provides a novel feasible avenue for constructing next-generation smart polarization-sensitive photodetector with low energy consumption.

Abstract

Novel 2D materials with low-symmetry structures exhibit great potential applications in developing monolithic polarization-sensitive photodetectors with small volume. However, owing to the fact that at least half of them presented a small anisotropic factor of ≈2, comprehensive performance of present polarization-sensitive photodetectors based on 2D materials is still lower than the practical application requirements. Herein, a self-driven photodetector with high polarization sensitivity using a broken-gap ReSe₂/SnSe₂ van der Waals heterojunction (vdWH) is demonstrated. Anisotropic ratio of the photocurrent (I _max/I _min) could reach 12.26 (635 nm, 179 mW cm⁻²). Furthermore, after a facile combination of the ReSe₂/SnSe₂ device with multilayer graphene (MLG), I _max/I _min of the MLG/ReSe₂/SnSe₂ can be further increased up to13.27, which is 4 times more than that of pristine ReSe₂ photodetector (3.1) and other 2D material photodetectors even at a bias voltage. Additionally, benefitting from the synergistic effect of unilateral depletion and photoinduced tunneling mechanism, the MLG/ReSe₂/SnSe₂ device exhibits a fast response speed (752/928 µs) and an ultrahigh light on/off ratio (10⁵). More importantly, MLG/ReSe₂/SnSe₂ device exhibits excellent potential applications in polarized imaging and polarization-coded optical communication with quaternary logic state without any power supply. This work provides a novel feasible avenue for constructing next-generation smart polarization-sensitive photodetector with low energy consumption.

Heterogeneous-structured MoB₂ (h-MoB₂) with amorphous shell and crystalline core, is prepared by solid phase molten salt method. The crystalline core encapsulates the honeycomb borophene within the two adjacent Mo atoms, and the amorphous shell accommodates more structural stress, guaranteeing superior lithium storage capacity and diffusion kinetics behavior.

Abstract

With the development of electric vehicles, exploiting anode materials with high capacity and fast charging capability is an urgent requirement for lithium-ion batteries (LIBs). Borophene, with the merits of high capacity, high electronic conductivity and fast diffusion kinetics, holds great potential as anode for LIBs. However, it is difficult to fabricate for the intrinsic electron-deficiency of boron atom. Herein, heterogeneous-structured MoB₂ (h-MoB₂) with amorphous shell and crystalline core, is prepared by solid phase molten salt method. As demonstrated, crystalline core can encapsulate the honeycomb borophene within two adjacent Mo atoms, and amorphous shell can accommodate more lithium ions to strengthen the lithium storage capacity and diffusion kinetics. According to theoretical calculations, the lithium adsorption energy in MoB₂ is about −2.7 eV, and the lithium diffusion energy barrier in MoB₂ is calculated to be 0.199 eV, guaranteeing the enhanced adsorption capability and fast diffusion kinetic behavior of Li⁺ ions. As a result, h-MoB₂ anode presents high capacity of 798 mAh g⁻¹ at 0.1 A g⁻¹, excellent rate performance of 183 mAh g⁻¹ at 5 A g⁻¹ and long-term cyclic stability for 1200 cycles. This work may inspire ideas for the fabrication of borophene analogs and two-dimensional metal borides.

2D nanomaterials have layered structures and exhibit excellent physical barrier properties. By introducing corrosion inhibitors and/or chemical modifications, organic coatings can be endowed with excellent self-healing properties. Furthermore, highly conductive nanomaterials can boost the cathodic protection performance of zinc-rich coatings and extend the lifespan of organic coatings in marine environments.

Abstract

2D nanomaterials, with extraordinary physical and chemical characteristics, have long been regarded as promising nanofillers in organic coatings for marine corrosion protection. The past decade has witnessed the high-speed progress of 2D nanomaterial-reinforced organic composite coatings, and plenty of breakthroughs have been achieved as yet. This review covers an in-depth and all-around outline of the up-to-date advances in 2D nanomaterial-modified organic coatings employed for the marine corrosion protection realm. Starting from a brief introduction to 2D nanomaterials, the preparation strategies and properties are illustrated. Subsequently, diverse protection models based on composite coatings for marine corrosion protection are also introduced, including physical barrier, self-healing, as well as cathodic protection, respectively. Furthermore, computational simulations and critical factors on the corrosion protection properties of composite coatings are clarified in detail. Finally, the remaining challenges and prospects for marine corrosion protection based on 2D nanomaterials reinforced organic coatings are highlighted.

This work reports the direct synthesis of novel 1D-defect-induced Co₄Te₇ superlattices on lattice-matched SrTiO₃(001) substrates, derived from 1T-CoTe₂ with spontaneously-evolved Te line defects in the upper Te layer. The detailed atomic structure of monolayer Co₄Te₇ and its distinctive electronic states related to flat bands are unveiled by scanning tunneling microscopy/spectroscopy (STM/STS).

Abstract

1D structures/patterns (e.g., line defect arrays, 1D Moiré patterns) embedded in 2D materials provide fascinating platforms for exploring versatile intriguing phenomena, for example, 1D Luttinger liquids and charge density waves (CDWs). Despite persistent efforts, incorporating periodic 1D patterns into 2D materials remains an ongoing pursuit. Herein, the direct preparation of monolayer 1D-defect-induced Co₄Te₇ superlattices (with periodic Te defect lines in the upper Te layer in 1T-CoTe₂) is reported, on lattice-matched SrTiO₃(001) (STO(001)) substrates via molecular beam epitaxy (MBE). Utilizing on-site scanning tunneling microscopy/spectroscopy (STM/STS) combined with density functional theory (DFT) calculations, the detailed atomic structure of monolayer Co₄Te₇ is identified, and its formation mechanisms from the synergistic effects of the Co/Te precursor ratio and adlayer-substrate interfacial coupling are uncovered. The potential flat-band feature of the monolayer Co₄Te₇ is also unveiled. This work should hereby offer valuable insights into the engineering of periodic 1D-defect patterns in 2D materials, as well as the atomic-scale structure and electronic property characterizations, thus paving ways for their intriguing property investigations.

npj 2D Materials and Applications, Published online: 18 March 2024; doi:10.1038/s41699-024-00460-1

Angle-resolved polarized Raman spectroscopy (ARPRS) of CrOCl under two typical and accurate configurations is systematically investigated for reliable crystal orientation. The phase-dependent ARPRS analysis emphasizes the importance of eliminating the analyzer angular deviation. 3D ARPRS reveals the high sensitivity of phase-dependent ARPRS in detecting LD-related effects. These results offer in-depth comprehension on precise controlling configurations for probing the photo-induced effects.

Abstract

Within angle-resolved polarized Raman spectroscopy (ARPRS) experiments, ensuring accurate configurations is crucial for obtaining reliable results, especially when dealing with anisotropic materials like CrOCl with anisotropic crystal structure and phonon properties. However, comprehensive understanding of phase-dependent ARPRS and the calibration of phase angle on anisotropic phonon modes from a crystallographic perspective are still lacking. Herein, detailed investigation on phase-dependent ARPRS and looked into the anisotropic photo-phonon interaction in CrOCl through in situ ARPRS is conducted. The Raman tensors and crystal orientation are acquired under two typical configurations. Phase-dependent ARPRS are thoroughly analyzed for the calibration of analyzer deviation. The anomalous anisotropic response in phase-dependent ARPRS is closely related to the intrinsic linear dichroism (LD) effect in CrOCl, which is further confirmed by angle-resolved polarization absorption. Moreover, the strong localized rules for various parallel energy bands of CrOCl have generated anomalous LD conversion effect near 470 nm, which expands the potential application of CrOCl in polarization-wavelength selected devices. This findings can offer in-depth comprehension on the anisotropic phonon physics of CrOCl and a feasible approach to utilize phase-dependent ARPRS for probing the undiscovered abundant photo-induced effects in low-symmetry 2D materials.

Nature Materials, Published online: 15 March 2024; doi:10.1038/s41563-024-01840-0

Orientation control is possible for deoxyribonucleic acid (DNA) in a hydrated state, and this affects to create cracks induced by drying. A method is for controlled crack generation in oriented DNA films by inducing mechanical fractures through organic solvent-induced dehydration (OSID). The straightforward approach devised for aligning cracks has the potential for use in various patterning applications.

Abstract

Crack is found on the soil when severe drought comes, which inspires the idea to rationalize patterning applications using dried deoxyribonucleic acid (DNA) film. DNA is one of the massively produced biomaterials in nature, showing the lyotropic liquid crystal (LC) phase in highly concentrated conditions. DNA nanostructures in the hydrated condition can be orientation controlled, which can be extended to make dryinginduced cracks. The controlled crack generation in oriented DNA films by inducing mechanical fracture through organic solvent-induced dehydration (OSID) using tetrahydrofuran (THF) is explored. The corresponding simulations show a strong correlation between the long axis of DNA due to the shrinkage during the dehydration and in the direction of crack propagation. The cracks are controlled by simple brushing and a 3D printing method. This facile way of aligning cracks will be used in potential patterning applications.

2D ferromagnetic M₃GeTe₂ (MGT, M = Ni/Fe) nanosheets with rich atomic Te vacancies are experimentally prepared as efficient electrocatalysts for the alkaline OER, where the generated atomic vacancies and surface metal-oxygen configurations are crucial for the highly improved intermediates adsorption, charge transfer kinetics, and overall reaction performances, enlighten the rational design of broad ferromagnetic materials for a wide range of electrocatalytic applications.

Abstract

In this work, 2D ferromagnetic M₃GeTe₂ (MGT, M = Ni/Fe) nanosheets with rich atomic Te vacancies (2D-MGT_v) are demonstrated as efficient OER electrocatalyst via a general mechanical exfoliation strategy. X-ray absorption spectra (XAS) and scanning transmission electron microscope (STEM) results validate the dominant presence of metal-O moieties and rich Te vacancies, respectively. The formed Te vacancies are active for the adsorption of OH* and O* species while the metal-O moieties promote the O* and OOH* adsorption, contributing synergistically to the faster oxygen evolution kinetics. Consequently, 2D-Ni₃GeTe_2v exhibits superior OER activity with only 370 mV overpotential to reach the current density of 100 mA cm⁻² and turnover frequency (TOF) value of 101.6 s⁻¹ at the overpotential of 200 mV in alkaline media. Furthermore, a 2D-Ni₃GeTe_2v-based anion-exchange membrane (AEM) water electrolysis cell (1 cm²) delivers a current density of 1.02 and 1.32 A cm⁻² at the voltage of 3 V feeding with 0.1 and 1 m KOH solution, respectively. The demonstrated metal-O coordination with abundant atomic vacancies for ferromagnetic M₃GeTe₂ and the easily extended preparation strategy would enlighten the rational design and fabrication of other ferromagnetic materials for wider electrocatalytic applications.

Nature Communications, Published online: 15 March 2024; doi:10.1038/s41467-024-46612-1