Precursor-Confined Chemical Vapor Deposition of 2D Single-Crystalline SexTe1-x Nanosheets for p-Type Transistors and Inverters.

Two-dimensional (2D) tellurium (Te) is emerging as a promising p-type candidate for constructing complementary metal-oxide-semiconductor (CMOS) architectures. However, its small bandgap leads to a high leakage current and a low on/off current ratio. Although alloying Te with selenium (Se) can tune its bandgap, thermally evaporated SexTe1-x thin films often suffer from grain boundaries and high-density defects. Herein, we introduce a precursor-confined chemical vapor deposition (CVD) method for synthesizing single-crystalline SexTe1-x alloy nanosheets. These nanosheets, with tunable compositions, are ideal for high-performance field-effect transistors (FETs) and 2D inverters. The preformation of Se-Te frameworks in our developed CVD method plays a critical role in the growth of SexTe1-x nanosheets with high crystallinity. Optimizing the Se composition resulted in a Se0.30Te0.70 nanosheet-based p-type FET with a large on/off current ratio of 4 × 105 and a room-temperature hole mobility of 120 cm2·V-1·s-1, being eight times higher than thermally evaporated SexTe1-x with similar composition and thickness. Moreover, we successfully fabricated an inverter based on p-type Se0.30Te0.70 and n-type MoS2 nanosheets, demonstrating a typical voltage transfer curve with a gain of 30 at an operation voltage of Vdd = 3 V.

Atomically Precise Chiral Metal Nanoclusters for Circularly Polarized Light Detection.

Circularly polarized light (CPL) detection is of great significance in various applications such as drug identification, sensing and imaging. Atomically precise chiral metal nanoclusters with intense circular dichroism (CD) signals are promising candidates for CPL detection, which can further facilitate device miniaturization and integration. Herein, we report the preparation of a pair of optically active chiral silver nanoclusters [Ag7(R/S-DMA)2(dpppy)3] (BF4)3 (R/S-Ag7) for direct CPL detection. The crystal structure and molecular formula of R/S-Ag7 clusters are confirmed by single-crystal x-ray diffraction and high-resolution mass spectrometry. R/S-Ag7 clusters exhibit strong CD spectra and CPL luminescence both in solution and solid states. When used as the photoactive materials in photodetectors, R/S-Ag7 enables effective discrimination between left-handed circularly polarized and right-handed circularly polarized light at 520 nm with short response time, high responsivity and considerable discrimination ratio. This study is the first report on using atomically precise chiral metal nanoclusters for CPL detection.

A 2D Heterostructure-Based Multifunctional Floating Gate Memory Device for Multimodal Reservoir Computing.

The demand for economical and efficient data processing has led to a surge of interest in neuromorphic computing based on emerging two-dimensional (2D) materials in recent years. As a rising van der Waals (vdW) p-type Weyl semiconductor with many intriguing properties, tellurium (Te) has been widely used in advanced electronics/optoelectronics. However, its application in floating gate (FG) memory devices for information processing has never been explored. Herein, an electronic/optoelectronic FG memory device enabled by Te-based 2D vdW heterostructure for multimodal reservoir computing (RC) is reported. When subjected to intense electrical/optical stimuli, the device exhibits impressive nonvolatile electronic memory behaviors including ≈108 extinction ratio, ≈100 ns switching speed, >4000 cycles, >4000-s retention stability, and nonvolatile multibit optoelectronic programmable characteristics. When the input stimuli weaken, the nonvolatile memory degrades into volatile memory. Leveraging these rich nonlinear dynamics, a multimodal RC system with high recognition accuracy of 90.77% for event-type multimodal handwritten digit-recognition is demonstrated.

Controlled Synthesis of Lead-Free Double Perovskite Colloidal Nanocrystals for Nonvolatile Resistive Memory Devices.

Although lead-free double perovskites such as Cs2AgBiBr6 have been widely explored, they still remain a daunting challenge for the controlled synthesis of lead-free double perovskite nanocrystals with highly tunable morphology and band structure. Here, we report the controlled synthesis of lead-free double perovskite colloidal nanocrystals including Cs2AgBiBr6 and Cs2AgInxBi1-xBr6 via a facile wet-chemical synthesis method for the fabrication of high-performance nonvolatile resistive memory devices. Cs2AgBiBr6 colloidal nanocrystals with well-defined cuboidal, hexagonal, and triangular morphologies are synthesized through a facile wet-chemical approach by tuning the reaction temperature from 150 to 190 °C. Further incorporating indium into Cs2AgBiBr6 to synthesize alloyed Cs2AgInxBi1-xBr6 nanocrystals not only can induce the indirect-to-direct bandgap transition with enhanced photoluminescence but also can improve its structural stability. After optimizing the active layers and device structure, the fabricated Ag/polymethylene acrylate@Cs2AgIn0.25Bi0.75Br6/ITO resistive memory device exhibits a low power consumption (the operating voltage is ∼0.17 V), excellent cycling stability (>10 000 cycles), and good synaptic property. Our study would enable the facile wet-chemical synthesis of lead-free double perovskite colloidal nanocrystals in a highly controllable manner for the development of high-performance resistive memory devices.

Constructing Chiral Covalent-Organic Frameworks for Circularly Polarized Light Detection.

The use of chiral covalent organic frameworks (COFs) as active elements in photodetectors to directly identify circularly polarized light (CPL) can meet the requirement of integration and miniaturization of the as-fabricated devices. Herein, the design and synthesis of two isoreticular chiral two-dimensional (2D) COFs (CityU-7 and CityU-8) by introducing photosensitive porphyrin-based amines (5,10,15,20-tetrakis(4-aminophenyl)porphyrin) to enhance the optical absorption and chiral aldehyde linkage (2,5-bis((S/R))-2-methylbutoxy)terephthalaldehyde) to engender chirality for direct CPL detection  are  reported. Their crystalline structures  were  confirmed by powder X-ray diffraction, Fourier-transform infrared spectroscopy, and low-dose transition electron microscopy. Employing both chiral COFs as the active layers in photodetectors, left-handed circularly (LHC) and right-handed circularly (RHC) polarized light at 405 nm can be well distinguishable with short response time, high responsivity, and satisfying detectivity. The study provides the first example on the design and synthesis of chiral COFs for direct detection of CPL.

Uncovering the Role of Crystal Phase in Determining Nonvolatile Flash Memory Device Performance Fabricated from MoTe2-Based 2D van der Waals Heterostructures.

Although the crystal phase of two-dimensional (2D) transition metal dichalcogenides (TMDs) has been proven to play an essential role in fabricating high-performance electronic devices in the past decade, its effect on the performance of 2D material-based flash memory devices still remains unclear. Here, we report the exploration of the effect of MoTe2 in different phases as the charge-trapping layer on the performance of 2D van der Waals (vdW) heterostructure-based flash memory devices, where a metallic 1T'-MoTe2 or semiconducting 2H-MoTe2 nanoflake is used as the floating gate. By conducting comprehensive measurements on the two kinds of vdW heterostructure-based devices, the memory device based on MoS2/h-BN/1T'-MoTe2 presents much better performance, including a larger memory window, faster switching speed (100 ns), and higher extinction ratio (107), than that of the device based on the MoS2/h-BN/2H-MoTe2 heterostructure. Moreover, the device based on the MoS2/h-BN/1T'-MoTe2 heterostructure also shows a long cycle (>1200 cycles) and retention (>3000 s) stability. Our study clearly demonstrates that the crystal phase of 2D TMDs has a significant impact on the performance of nonvolatile flash memory devices based on 2D vdW heterostructures, which paves the way for the fabrication of future high-performance memory devices based on 2D materials.

Electronic/Optoelectronic Memory Device Enabled by Tellurium-based 2D van der Waals Heterostructure for in-Sensor Reservoir Computing at the Optical Communication Band.

Although 2D materials are widely explored for data storage and neuromorphic computing, the construction of 2D material-based memory devices with optoelectronic responsivity in the short-wave infrared (SWIR) region for in-sensor reservoir computing (RC) at the optical communication band still remains a big challenge. In this work, an electronic/optoelectronic memory device enabled by tellurium-based 2D van der Waals (vdW) heterostructure is reported, where the ferroelectric CuInP2 S6 and tellurium channel endow this device with both the long-term potentiation/depression by voltage pulses and short-term potentiation by 1550 nm laser pulses (a typical wavelength in the conventional fiber optical communication band). Leveraging the rich dynamics, a fully memristive in-sensor RC system that can simultaneously sense, decode, and learn messages transmitted by optical fibers is demonstrated. The reported 2D vdW heterostructure-based memory featuring both the long-term and short-term memory behaviors using electrical and optical pulses in SWIR region has not only complemented the wide spectrum of applications of 2D materials family in electronics/optoelectronics but also paves the way for future smart signal processing systems at the edge.

Bright and Efficient Light-Emitting Devices Based on 2D Transition Metal Dichalcogenides.

2D monolayer transition metal dichalcogenides (TMDCs) show great promise for the development of next-generation light-emitting devices owing to their unique electronic and optoelectronic properties. The dangling-bond-free surface and direct-bandgap structure of monolayer TMDCs allow for near-unity photoluminescence quantum efficiencies. The excellent mechanical and optical characteristics of 2D TMDCs offer great potential to fabricate TMDC-based light-emitting diodes (LEDs) featuring good flexibility and transparency. Great progress has been made in the fabrication of bright and efficient LEDs with varying device structures. In this review, the aim is to provide a comprehensive summary of the state-of-the-art progress made in the construction of bright and efficient LEDs based on 2D TMDCs. After a brief introduction to the research background, the preparation of 2D TMDCs used for LEDs is briefly discussed. The requirements and the corresponding challenges to achieve bright and efficient LEDs based on 2D TMDCs are introduced. Thereafter, various strategies to enhance the brightness of monolayer 2D TMDCs are described. Following that, the carrier-injection schemes enabling bright and efficient TMDC-based LEDs along with the device performance are summarized. Finally, the challenges and future prospects regarding the accomplishment of TMDC-LEDs with ultimate brightness and efficiency are discussed.

Layered double hydroxide-based nanomaterials for biomedical applications.

Against the backdrop of increased public health awareness, inorganic nanomaterials have been widely explored as promising nanoagents for various kinds of biomedical applications. Layered double hydroxides (LDHs), with versatile physicochemical advantages including excellent biocompatibility, pH-sensitive biodegradability, highly tunable chemical composition and structure, and ease of composite formation with other materials, have shown great promise in biomedical applications. In this review, we comprehensively summarize the recent advances in LDH-based nanomaterials for biomedical applications. Firstly, the material categories and advantages of LDH-based nanomaterials are discussed. The preparation and surface modification of LDH-based nanomaterials, including pristine LDHs, LDH-based nanocomposites and LDH-derived nanomaterials, are then described. Thereafter, we systematically describe the great potential of LDHs in biomedical applications including drug/gene delivery, bioimaging diagnosis, cancer therapy, biosensing, tissue engineering, and anti-bacteria. Finally, on the basis of the current state of the art, we conclude with insights on the remaining challenges and future prospects in this rapidly emerging field.

Defect engineering of layered double hydroxide nanosheets as inorganic photosensitizers for NIR-III photodynamic cancer therapy.

Although two-dimensional (2D) layered double hydroxides (LDHs) have been widely used as efficient nanoagents for biological diagnosis and treatment, they have been found to be inert as photosensitizers (PSs) for photodynamic therapy (PDT). Herein, we report the defect engineering of ultrathin 2D CoMo-LDH and NiMo-LDH nanosheets as highly active inorganic PSs for PDT in the third near-infrared (NIR-III) window. Hydrothermal-synthesized 2D CoMo-LDH and NiMo-LDH nanosheets are etched via a simple acid treatment to obtain defect-rich CoMo-LDH and NiMo-LDH nanosheets. Importantly, the defect-rich CoMo-LDH nanosheets exhibit much higher activity (~97 times) for generation of reactive oxygen species than that of the pristine CoMo-LDH nanosheets under a NIR-III 1567 nm laser irradiation. Therefore, after modification with polyethylene glycol, the defect-rich CoMo-LDH nanosheets can be used as an efficient inorganic PS for PDT to efficiently induce cancer cells apoptosis in vitro and eradicate tumors in vivo under 1567 nm laser irradiation.

Growth of Tellurium Nanobelts on h-BN for p-type Transistors with Ultrahigh Hole Mobility.

The lack of stable p-type van der Waals (vdW) semiconductors with high hole mobility severely impedes the step of low-dimensional materials entering the industrial circle. Although p-type black phosphorus (bP) and tellurium (Te) have shown promising hole mobilities, the instability under ambient conditions of bP and relatively low hole mobility of Te remain as daunting issues. Here we report the growth of high-quality Te nanobelts on atomically flat hexagonal boron nitride (h-BN) for high-performance p-type field-effect transistors (FETs). Importantly, the Te-based FET exhibits an ultrahigh hole mobility up to 1370 cm2 V-1 s-1 at room temperature, that may lay the foundation for the future high-performance p-type 2D FET and metal-oxide-semiconductor (p-MOS) inverter. The vdW h-BN dielectric substrate not only provides an ultra-flat surface without dangling bonds for growth of high-quality Te nanobelts, but also reduces the scattering centers at the interface between the channel material and the dielectric layer, thus resulting in the ultrahigh hole mobility .

Intercalation-Activated Layered MoO3 Nanobelts as Biodegradable Nanozymes for Tumor-Specific Photo-Enhanced Catalytic Therapy.

The existence of natural van der Waals gaps in layered materials allows them to be easily intercalated with varying guest species, offering an appealing strategy to optimize their physicochemical properties and application performance. Herein, we report the activation of layered MoO3 nanobelts via aqueous intercalation as an efficient biodegradable nanozyme for tumor-specific photo-enhanced catalytic therapy. The long MoO3 nanobelts are grinded and then intercalated with Na+ and H2 O to obtain the short Na+ /H2 O co-intercalated MoO3-x (NH-MoO3-x ) nanobelts. In contrast to the inert MoO3 nanobelts, the NH-MoO3-x nanobelts exhibit excellent enzyme-mimicking catalytic activity for generation of reactive oxygen species, which can be further enhanced by the photothermal effect under a 1064 nm laser irradiation. Thus, after bovine serum albumin modification, the NH-MoO3-x nanobelts can efficiently kill cancer cells in vitro and eliminate tumors in vivo facilitating with 1064 nm laser irradiation.