Gate-Coupling-Enabled Robust Hysteresis for Nonvolatile Memory and Programmable Rectifier in Van der Waals Ferroelectric Heterojunctions

Ferroelectric field-effect transistors (FeFETs) are one of the most interesting ferroelectric devices; however, they, usually suffer from low interface quality. The recently discovered 2D layered ferroelectric materials, combining with the advantages of van der Waals heterostructures (vdWHs), may be promising to fabricate high-quality FeFETs with atomically thin thickness. Here, dual-gated 2D ferroelectric vdWHs are constructed using MoS₂, hexagonal boron nitride (h-BN), and CuInP₂S₆ (CIPS), which act as a high-performance nonvolatile memory and programmable rectifier. It is first noted that the insertion of h-BN and dual-gated coupling device configuration can significantly stabilize and effectively polarize ferroelectric CIPS. Through this design, the device shows a record-high performance with a large memory window, large on/off ratio (10⁷), ultralow programming state current (10⁻¹³ A), and long-time endurance (10⁴ s) as nonvolatile memory. As for programmable rectifier, a wide range of gate-tunable rectification behavior is observed. Moreover, the device exhibits a large rectification ratio (3 × 10⁵) with stable retention under the programming state. This demonstrates the promising potential of ferroelectric vdWHs for new multifunctional ferroelectric devices.     

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD‐grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.     

Multibit Optoelectronic Memory in Top‐Floating‐Gated van der Waals Heterostructures

Nonvolatile memories based on van der Waals heterostructures have been proved to be promising candidates for next‐generation data storage devices. However, little attention has been focused on the structure with separated floating and control gates (the floating gates and control gates distribute at the different side of the channels), which were recently predicted to be capable of further improving device performance. Here, nonvolatile multibit optoelectronic memories are demonstrated using MoS₂, hexagonal boron nitride (h‐BN), and graphene in a top‐floating‐gated structure. With separated top graphene floating gate, the devices show a large memory window (≈95 V) via sweeping gate voltage from 80 to ?80 V, a high on/off ratio (≈10⁶) with an ultralow dark current (≈10^?14 A), as well as excellent retention characteristic (≈10⁴ s) and cyclic endurance. In addition, these devices can also be erased by a laser illumination with broadband spectrum after being electrically programmed. For the multilevel storage property, 7/6 stages controlled by different electrical operations, and 13/6/3 stages by different laser pulse illuminations are gained. The obtained results show a promising performance for nonvolatile optoelectronic memory using a top‐floating‐gated structure.     

Ce Site in Amorphous Iron Oxyhydroxide Nanosheet toward Enhanced Electrochemical Water Oxidation

Iron oxyhydroxide has been considered an auspicious electrocatalyst for the oxygen evolution reaction (OER) in alkaline water electrolysis due to its suitable electronic structure and abundant reserves. However, Fe-based materials seriously suffer from the tradeoff between activity and stability at a high current density above 100 mA cm⁻². In this work, the Ce atom is introduced into the amorphous iron oxyhydroxide (i.e., CeFeO_xH_y) nanosheet to simultaneously improve the intrinsic electrocatalytic activity and stability for OER through regulating the redox property of iron oxyhydroxide. In particular, the Ce substitution leads to the distorted octahedral crystal structure of CeFeO_xH_y, along with a regulated coordination site. The CeFeO_xH_y electrode exhibits a low overpotential of 250 mV at 100 mA cm⁻² with a small Tafel slope of 35.1 mVdec⁻¹. Moreover, the CeFeO_xH_y electrode can continuously work for 300 h at 100 mA cm⁻². When applying the CeFeO_xH_y nanosheet electrode as the anode and coupling it with the platinum mesh cathode, the cell voltage for overall water splitting can be lowered to 1.47 V at 10 mA cm⁻². This work offers a design strategy for highly active, low-cost, and durable material through interfacing high valent metals with earth-abundant oxides/hydroxides.     

Dual Functional Titanium Hydride Particles for Anti-Ultraviolet and Anti-Oxidant Applications

TiO₂ is a typical anti-ultraviolet agent in sunscreen cream, but it suffers from a lack of anti-oxidant activity for scavenging reactive oxygen species. Other traditional sunscreen ingredients, such as C₆₀ and ferulic acid, have moderate antioxidant and UV protection capabilities, and the products do not provide further UV protection. Herein, titanium hydride (TiH_1.97) particles with dual functions of anti-oxidant and anti-ultraviolet are presented as an active material to remove the highly oxidizing and harmful hydroxide free radicals from the skin surface. Owing to the active hydrogen in TiH₂, it shows great potential to eliminate hydroxyl radicals, and the rate is roughly proportional to the exposed surface area (50% /20 min). Together with the biocompatibility of the in situ formed TiO₂, which has a strong ability to absorb ultraviolet light during OH radical scavenging. At the same time, adding titanium hydride to sunscreen also enhances sun protection. The SPF value of sunscreen containing 1% titanium hydride and 20% titanium dioxide can reach 31.8, which exceeds the 20% titanium dioxide (15) by more than twofold. Therefore, titanium hydride with anti-ultraviolet and antioxidant functions can be regarded as a promising and safe ingredient for sunscreen cream.     

Growth and Raman Scattering Investigation of a New 2D MOX Material: YbOCl

MOX (M = Fe, Co, Mn, Cr, Lanthanide, or Actinide metals; O = oxygen, X = F, Cl, Br, I), an emerging type of 2D layered materials, have been theoretically predicted to possess unique electronic and magnetic properties. However, 2D MOX have rarely been investigated. Herein, for the first time, ultrathin high‐quality ytterbium oxychloride (YbOCl) single crystals are successfully synthesized via an atmospheric pressure chemical vapor deposition method. Both theoretical simulations and experimental measurements are utilized to systematically investigate the Raman properties of 2D YbOCl nanosheets. The experimentally observed E_g mode at 85.53 cm^?1 and A_1g mode at 138.17 cm^?1 demonstrate a good match to the results from density functional theory calculations. Furthermore, the temperature‐dependent and thickness‐dependent Raman scattering spectra reveal the adjacent layers in YbOCl nanosheets show a relatively weak van der Waals interaction. Additionally, the polarized‐dependent Raman scattering spectra show the intensity of A_1g mode exhibits twofold patterns while the intensity of the E_g mode remains constant as the rotation angle changes. These findings could provide the first‐hand experimental information about the 2D YbOCl crystals.     

Anti‐Ambipolar Transport with Large Electrical Modulation in 2D Heterostructured Devices

Van der Waals materials and their heterostructures provide a versatile platform to explore new device architectures and functionalities beyond conventional semiconductors. Of particular interest is anti‐ambipolar behavior, which holds potentials for various digital electronic applications. However, most of the previously conducted studies are focused on hetero‐ or homo‐ p–n junctions, which suffer from a weak electrical modulation. Here, the anti‐ambipolar transport behavior and negative transconductance of MoTe₂ transistors are reported using a graphene/h‐BN floating‐gate structure to dynamically modulate the conduction polarity. Due to the asymmetric electrical field regulating effect on the recombination and diffusion currents, the anti‐ambipolar transport and negative transconductance feature can be systematically controlled. Consequently, the device shows an unprecedented peak resistance modulation factor (≈5 × 10³), and effective photoexcitation modulation with distinct threshold voltage shift and large photo on/off ratio (≈10⁴). Utilizing this large modulation effect, the voltage‐transfer characteristics of an inverter circuit variant are further studied and its applications in Schmitt triggers and multivalue output are further explored. These properties, in addition to their proven nonvolatile storage, suggest that such 2D heterostructured devices display promising perspectives toward future logic applications.