Jiuxiang Dai

Inspired by the uncontrolled recrystallization phenomenon of polycrystalline films, a general spatial confinement recrystallization method is designed to obtain uniform and thickness-controllable organic semiconductor single crystals (OSSCs) in 10 s by applying longitudinal pressure. This method provides a strategy for rapid preparation of high-quality OSSCs and high-performance organic field-effect transistors.

Abstract

Organic semiconductor single crystals (OSSCs) are ideal materials for studying the intrinsic properties of organic semiconductors (OSCs) and constructing high-performance organic field-effect transistors (OFETs). However, there is no general method to rapidly prepare thickness-controllable and uniform single crystals for various OSCs. Here, inspired by the recrystallization (a spontaneous morphological instability phenomenon) of polycrystalline films, a spatial confinement recrystallization (SCR) method is developed to rapidly (even at several second timescales) grow thickness-controllable and uniform OSSCs in a well-controlled way by applying longitudinal pressure to tailor the growth direction of grains in OSCs polycrystalline films. The relationship between growth parameters including the growth time, temperature, longitudinal pressure, and thickness is comprehensively investigated. Remarkably, this method is applicable for various OSCs including insoluble and soluble small molecules and polymers, and can realize the high-quality crystal array growth. The corresponding 50 dinaphtho[2,3-b:2″,3″-f]thieno[3,2-b]thiophene (DNTT) single crystals coplanar OFETs prepared by the same batch have the mobility of 4.1 ± 0.4 cm² V⁻¹ s⁻¹, showing excellent uniformity. The overall performance of the method is superior to the reported methods in term of growth rate, generality, thickness controllability, and uniformity, indicating its broad application prospects in organic electronic and optoelectronic devices.

This review provides a summary and discussion on the recent significant progress of porous 2D materials. The synthesis of these porous 2D materials and their corresponding applications are introduced and discussed, followed by a discussion on the challenges in the porous structure of the materials themself, materials synthesis, and future developments of expanding the applications of porous 2D materials.

Abstract

Two-Dimensional (2D) materials have attracted immense attention in recent years. These materials have found their applications in various fields, such as catalysis, adsorption, energy storage, and sensing, as they exhibit excellent physical, chemical, electronic, photonic, and biological properties. Recently, researchers have focused on constructing porous structures on 2D materials. Various strategies, such as chemical etching and template-based methods, for the development of surface pores are reported, and the porous 2D materials fabricated over the years are used to develop supercapacitors and energy storage devices. Moreover, the lattice structure of the 2D materials can be modulated during the construction of porous structures to develop 2D materials that can be used in various fields such as lattice defects in 2D nanomaterials for enhancing biomedical performances. This review focuses on the recently developed chemical etching, solvent thermal synthesis, microwave combustion, and template methods that are used to fabricate porous 2D materials. The application prospects of the porous 2D materials are summarized. Finally, the key scientific challenges associated with developing porous 2D materials are presented to provide a platform for developing porous 2D materials.

In flexible electronic devices, the weak van der Waals interface between graphene and stretchable polymeric substrates makes graphene prone to structural degradation from dynamic mechanical loads. To mitigate this, a polymer capping layer is spin-coated onto graphene, effectively encapsulating it. Results demonstrate that capping layers significantly prevent fatigue damage in graphene, even after enduring 100 cycles at a 10% strain.

In applications such as flexible electronic devices, graphene, and other 2D materials are frequently in contact with stretchable polymeric substrates. The interface between 2D materials and polymers is dominated by weak van der Waals forces and can eventually degrade due to the frequent dynamic mechanical loads that these devices experience. This can lead to significant local delamination and shear fracture of the 2D materials. Using the polydimethylsiloxane (PDMS) encapsulation method, it is shown that the damage in graphene is significantly mitigated when it is capped during dynamic loading. To track the spread of damage in both encapsulated and nonencapsulated graphene, in situ, cyclic loading is performed. The fundamental process driving this substantial reduction in damage propagation in the 2D lattice is explained by the conventional shear lag model. It is also observed that softer PDMS substrate and capping layer completely mitigate the fatigue damage in graphene for 100 cycles at 10% applied fatigue strain.

The successful induction and electrical detection of a methane oxidation reaction at room temperature using IrO₂ nanosheets, a 2D form of IrO₂, are reported. A clear decrease in electrical resistance upon exposure to methane is observed by using atomically thin IrO₂ nanosheet films. This electrical signal is corroborated by ambient-pressure X-ray photoemission spectroscopy and Raman scattering spectroscopy.

Abstract

Activation of the CH bond is the first step in converting methane into valuable chemicals. Herein, the successful induction and electrical detection of a methane oxidation reaction are reported at room temperature using IrO₂ nanosheets, a 2D form of IrO₂. A clear decrease in electrical resistance upon exposure to methane is observed by using atomically thin IrO₂ nanosheet films. The resistance decrease disappears upon simultaneous exposure to oxygen, suggesting that methane is oxidized by consuming the lattice oxygen of the IrO₂ nanosheets and that the oxygen vacancies generated are recovered by oxygen in the atmosphere. The resistance decrease is observed even at 300 K, indicating the high methane oxidation ability of the IrO₂ nanosheets. These results are confirmed by a shift of the Ir 4f peaks in ambient pressure X-ray photoelectron spectra. Furthermore, deposition of amorphous carbon, that is, methane oxidation products, on IrO₂ nanosheets is also confirmed by Raman scattering spectroscopy after prolonged methane exposure at high temperatures in the absence of oxygen. This study demonstrates the ability of IrO₂ nanosheets to oxidize methane at least down to 300 K and is an important example of the usefulness and simplicity of chemical reaction monitoring using electrical resistance changes.

Light: Science & Applications, Published online: 02 June 2023; doi:10.1038/s41377-023-01184-5

Single nitrogen vacancy created in hexagonal boron nitride by electron irradiation at 60 keV using an ultra-high vacuum scanning transmission electron microscope. In this filtered and colored medium-angle annular dark field image, the nitrogen atoms show brighter contrast than boron due to their greater atomic number, and the vacancy is visible as triangular dark contrast on the top left.

Abstract

Understanding electron irradiation effects is vital not only for reliable transmission electron microscopy characterization, but increasingly also for the controlled manipulation of 2D materials. The displacement cross sections of monolayer hexagonal boron nitride (hBN) are measured using aberration-corrected scanning transmission electron microscopy in near ultra-high vacuum at primary beam energies between 50 and 90 keV. Damage rates below 80 keV are up to three orders of magnitude lower than previously measured at edges under poorer residual vacuum conditions, where chemical etching appears to dominate. Notably, it is possible to create single vacancies in hBN using electron irradiation, with boron almost twice as likely as nitrogen to be ejected below 80 keV. Moreover, any damage at such low energies cannot be explained by elastic knock-on, even when accounting for the vibrations of the atoms. A theoretical description is developed to account for the lowering of the displacement threshold due to valence ionization resulting from inelastic scattering of probe electrons, modeled using charge-constrained density functional theory molecular dynamics. Although significant reductions are found depending on the constrained charge, quantitative predictions for realistic ionization states are currently not possible. Nonetheless, there is potential for defect-engineering of hBN at the level of single vacancies using electron irradiation.

In this work, a 2D WSe₂-based 6 × 6 retinal perception array that demonstrates an artificial vision system integrating the sense and memory visual information, is reported. In addition, highly linear symmetric synaptic plasticity can be achieved based on the modulation of carrier types in WSe₂ with different thicknesses, facilitating the high level of training accuracy for optical neural networks.

Abstract

Machine vision systems that capture images for visual inspection and recognition tasks must be able to perceive, memorize, and compute any color scene. To achieve this, most of the current visual systems use circuits and algorithms which may reduce efficiency and increase complexity. Herein, a 2D semiconductor tungsten diselenide (WSe₂)-based phototransistor that successfully demonstrates an artificial vision system integrating the processing capability of visual information sensing memory, is reported. Furthermore, based on a 6 × 6 fabricated retinal perception array, artificial visual information sensing memory and processing system are proposed to perform image recognition tasks, which can avoid the time delay and energy consumption caused by data conversion and movement. On the other hand, highly linear symmetric synaptic plasticity can be achieved based on the modulation of carrier types in WSe₂ transistors with different thicknesses, facilitating the high level of training and inference accuracy for artificial neural networks. Last, through training and inference simulations, the feasibility of the hybrid synapses for optical neural networks (ONN) is demonstrated.

npj 2D Materials and Applications, Published online: 01 June 2023; doi:10.1038/s41699-023-00404-1

Abstract

Two-dimensional (2D) materials with reversible phase transformation are appealing for their rich physics and potential applications in information storage. However, up to now, reversible phase transitions in 2D materials that can be driven by facile nondestructive methods, such as temperature, are still rare. Here, we introduce ultrathin Cu₉S₅ crystals grown by chemical vapor deposition (CVD) as an exemplary case. For the first time, their basic electrical properties were investigated based on Hall measurements, showing a record high hole carrier density of ∼ 10²² cm⁻³ among 2D semiconductors. Besides, an unusual and repeatable conductivity switching behavior at ∼ 250 K were readily observed in a wide thickness range of CVD-grown Cu₉S₅ (down to 2 unit-cells). Confirmed by in-situ selected area electron diffraction, this unusual behavior can be ascribed to the reversible structural phase transition between the room-temperature hexagonal β phase and low-temperature β′ phase with a superstructure. Our work provides new insights to understand the physical properties of ultrathin Cu₉S₅ crystals, and brings new blood to the 2D materials family with reversible phase transitions.

Nature Materials, Published online: 01 June 2023; doi:10.1038/s41563-023-01548-7

Abstract

Bilayer graphene provides a versatile platform for exploring a variety of intriguing phenomena and shows much promise for applications in electronics, optoelectronics, etc. Controlled growth of large-area bilayer graphene is therefore highly desired yet still suffers from a slow growth rate and poor layer uniformity. Meanwhile, graphene wrinkles, including folds and ripples, form during cooling due to the thermal contraction mismatch between graphene and the metal substrates, and have been far from suppressed or eliminated, especially in bilayer graphene, which would greatly degrade the extraordinary properties of graphene. Here we report the ultrafast growth of wafer-scale fold-free bilayer graphene by chemical vapor deposition. Through well-tuning the alloy thickness and strain regulation of the single-crystal CuNi(111)/sapphire, the full coverage of a 2-inch fold-free bilayer graphene wafer via mainly isothermal segregation has been achieved as fast as 30 s. The tensile-strained CuNi(111) film reduces the thermal contraction mismatch and suppresses the formation of graphene folds during cooling, which is directly observed through in situ optical microscopy. The ultraflat bilayer graphene exhibits wafer-scale uniformity in electrical performance and enhanced mechanical property comparable to the exfoliated ones. Our results offer a promising route for large-scale production of bilayer graphene and enable its various applications.

Nature, Published online: 31 May 2023; doi:10.1038/s41586-023-06011-w

Quasi-1D antimony chalcogenides are promising light-absorbing materials that demand a (001) crystallographic orientation for optimal performance in device applications. This study links substrate nanostructure to thin film orientation in quasi-1D materials. Here, exceptionally highly (001) oriented Sb₂Se₃ layers are achieved using substrate nanostructure engineering to “filter out” undesirable (hk0) crystals, and select (001), in the early growth stage.

Abstract

Antimony chalcogenide, Sb₂X₃ (X = S, Se), applications greatly benefit from efficient charge transport along covalently bonded (001) oriented (Sb₄X₆) _n ribbons, making thin film orientation control highly desirable – although particularly hard to achieve experimentally. Here, it is shown for the first time that substrate nanostructure plays a key role in driving the growth of (001) oriented antimony chalcogenide thin films. Vapor Transport Deposition of Sb₂Se₃ thin films is conducted on ZnO substrates whose morphology is tuned between highly nanostructured and flat. The extent of Sb₂Se₃ (001) orientation is directly correlated to the degree of substrate nanostructure. These data showcase that nanostructuring a substrate is an effective tool to control the orientation and morphology of Sb₂Se₃ films. The optimized samples demonstrate high (001) crystallographic orientation. A growth mechanism for these films is proposed, wherein the substrate physically restricts the development of undesirable crystallographic orientations. It is shown that the surface chemistry of the nanostructured substrates can be altered and still drive the growth of (001) Sb₂Se₃ thin films – not limiting this phenomenon to a particular substrate type. Insights from this work are expected to guide the rational design of Sb₂X₃ thin film devices and other low-dimensional crystal-structured materials wherein performance is intrinsically linked to morphology and orientation.

Nature Synthesis, Published online: 30 May 2023; doi:10.1038/s44160-023-00338-y

Abstract

Two-dimensional (2D) tribotronic devices have been successfully involved in electromechanical modulation for channel conductance and applied in intelligent sensing system, touch screen, and logic gates. Ambipolar transistors and corresponding complementary inverters based on one type of semiconductors are highly promising due to the facile fabrication process and readily tunable polarity. Here, we demonstrate an ambipolar tribotronic transistor of molybdenum ditelluride (MoTe₂), which shows typical ambipolar transport properties modulated by triboelectric potential. It is comprised of a MoTe₂ transistor and a lateral sliding triboelectric nanogenerator (TENG). The induced triboelectric potential by Maxwell’s displacement current (a driving force for TENG) can readily modulate the transport properties of both electrons and holes in MoTe₂ channel and effectively drive the transistor. High performance tribotronic properties have been achieved, including low cutoff current below 1 pA·µm⁻¹ and high current on/off ratio of ∼ 10³ for holes and electrons dominated transports. The working mechanism on how to achieve tribotronic ambipolarity is discussed in detail. A complementary tribotronic inverter based on single flake of MoTe₂ is also demonstrated with low power consumption and high stability. This work presents an active approach to efficiently modulate semiconductor devices and logic circuits based on 2D materials through external mechanical signal, which has great potential in human–machine interaction, intelligent sensor, and other wearable devices.