GaS:WS2 Heterojunctions for Ultrathin Two-Dimensional Photodetectors with Large Linear Dynamic Range across Broad Wavelengths.

Two-dimensional (2D) photodetectors based on photovoltaic effect or photogating effect can hardly achieve both high photoresponsivity and large linear dynamic range at the same time, which greatly limits many practical applications such as imaging sensors. Here, the conductive-sensitizer strategy, a general design for improving photoresponsivity and linear dynamic range in 2D photodetectors is provided and experimentally demonstrated on vertically stacked bilayer WS2/GaS0.87 under a parallel circuit mode. Owing to successful band alignment engineering, the isotype type-II heterojunction enables efficient charge carrier transfer from WS2, the high-mobility sensitizer, to GaS0.87, the low-mobility channel, under illumination from a broad visible spectrum. The transferred electron charges introduce a reverse electric field which efficiently lowers the band offset between the two materials, facilitating a transition from low-mobility photocarrier transport to high-mobility photocarrier transport with increasing illumination power. We achieved a large linear dynamic range of 73 dB as well as a high and constant photoresponsivity of 13 A/W under green light. X-ray photoelectron spectroscopy, cathodoluminescence, and Kelvin probe force microscopy further identify the key role of defects in monolayer GaS0.87 in engineering the band alignment with monolayer WS2. This work proposes a design route based on band and interface modulation for improving performance of 2D photodetectors and provides deep insights into the important role of strong interlayer coupling in offering heterostructures with desired properties and functions.

Atomistic Mechanics of Torn Back Folded Edges of Triangular Voids in Monolayer WS2.

Triangular nanovoids in 2D materials transition metal dichalcogenides have vertex points that cause stress concentration and lead to sharp crack propagation and failure. Here, the atomistic mechanics of back folding around triangular nanovoids in monolayer WS2 sheets is examined. Combining atomic-resolution images from annular dark-field scanning transmission electron microscopy with reactive molecular modelling, it is revealed that the folding edge formation has statistical preferences under geometric conditions based on the orientation mismatch. It is further investigated how loading directions and strong interlayer friction, interplay with WS2 lattice's crack preference, govern the deformation and fracture pattern around folding edges. These results provide fundamental insights into the combination of fracture and folding in flexible monolayer crystals and the resultant Moiré lattices.

Ghrelin inhibits IKKß/NF-κB activation and reduces pro-inflammatory cytokine production in pancreatic acinar AR42J cells treated with cerulein.

BACKGROUND: Previous studies have provided conflicting results regarding whether the serum ghrelin concentration can reflect the severity of acute pancreatitis (AP). The present study examined the correlation between the serum ghrelin concentration and AP severity in animal models and investigated whether altered ghrelin expression in pancreatic acinar cells influences IKKß/NF-κB signaling and pro-inflammatory cytokine production. METHODS: Mild or severe AP was induced in rats by intraperitoneal injection of cerulein or retrograde cholangiopancreatic duct injection of sodium taurocholate, respectively. After successful model induction, serum ghrelin, tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) concentrations were determined by enzyme-linked immunosorbent assay, and IKKß/NF-κB activation was assessed by immunohistochemistry. Subsequently, stable overexpression or knockdown of ghrelin in AR42J cells was achieved by lentiviral transfection. After transfected cells and control cells were treated with cerulein for 24 h, the TNF-α and IL-1ß levels in the supernatants were determined by enzyme-linked immunosorbent assay, and the expression levels of p-p65, IKKß, and p-IKKß were detected by Western blotting. RESULTS: In rat AP models, AP severity was correlated with increased IKKß/NF-κB activation, pro-inflammatory cytokine production, and ghrelin secretion. The levels of pro-inflammatory cytokines TNF-α and IL-1ß as well as IKKß/NF-κB signaling activity were increased upon knockdown of ghrelin in the AP acinar cell model and decreased with ghrelin overexpression. CONCLUSIONS: Serum ghrelin is related to the severity of AP. Ghrelin may play a protective role in the pathogenesis of AP by inhibiting the pro-inflammatory cytokines and the activation of the IKKß/NF-κB signaling pathway.

Phase Variations and Layer Epitaxy of 2D PdSe2 Grown on 2D Monolayers by Direct Selenization of Molecular Pd Precursors.

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe2 and Pd2Se3 are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensors. This research presents a feasible approach to synthesize PdSe2 and Pd2Se3 with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS2. We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by low-temperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd2Se3 phase, to continuous few-layer PdSe2 films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe2 with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe2 and monolayer Pd2Se3 are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe2 and underlying 2D substrate was also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.

Controlling Defects in Continuous 2D GaS Films for High-Performance Wavelength-Tunable UV-Discriminating Photodetectors.

A chemical vapor deposition method is developed for thickness-controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS0.87 and stoichiometric GaS. The unique degradation mechanism of GaS0.87 with X-ray photoelectron spectroscopy and annular dark-field scanning transmission electron microscopy is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo-induced oxidation at defects. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis-to-UV to UV-discriminating. The stoichiometric GaS is suitable for large-scale, UV-sensitive, high-performance photodetector arrays for information encoding under large vis-light noise, with short response time (<66 ms), excellent UV photoresponsivity (4.7 A W-1 for trilayer GaS), and 26-times increase of signal-to-noise ratio compared with small-bandgap 2D semiconductors. By comprehensive characterizations from atomic-scale structures to large-scale device performances in 2D semiconductors, the study provides insights into the role of defects, the importance of neglected material-quality control, and how to enhance device performance, and both layer-controlled defective GaS0.87 and stoichiometric GaS prove to be promising platforms for study of novel phenomena and new applications.

Ultrathin All-2D Lateral Graphene/GaS/Graphene UV Photodetectors by Direct CVD Growth.

UV-sensitive lateral all-two-dimensional (2D) photodetecting devices are produced by growing the large band gap layered GaS between graphene electrode pairs directly using chemical vapor deposition methods. The use of prepatterned graphene electrode pairs on the Si wafer enables more than 200 devices to be fabricated simultaneously. We show that the surface chemistry of the substrate during GaS leads to selective growth in graphene gaps, forming the lateral heterostructures, rather than on the surface of graphene. The graphene/GaS/graphene lateral photodetecting devices are demonstrated to be sensitive to UV light only, with no measurable response to visible light. Furthermore, we demonstrate UV-band discrimination in photosensing, with measured photocurrents only produced for middle-UV and not for near-UV wavelength regions. The detection limit could reach down to 2.61 µW/cm2 with a photoresponsivity as high as 11.7 A/W and a photo gain of 53.7 under 270 nm excitation. Gate-dependent modulation of the photocurrent is also demonstrated. The photodetectors exhibit long-term stability and reproducible ON-OFF switching behavior, with a response time lower than 60 ms. These results provide insights into how ultrathin UV sensing devices can be created using only 2D materials by exploiting large band gap 2D semiconductors such as GaS.

Atomically Sharp Dual Grain Boundaries in 2D WS2 Bilayers.

It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS2 can be atomically sharp, where top and bottom layer GBs are located within sub-nanometer distances of each other. This expands the current knowledge of GBs in 2D bilayer crystals, beyond the established large overlapping GB types typically formed in chemical vapor deposition growth, to now include atomically sharp dual bilayer GBs. By using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging, different atomic structures in the dual GBs are distinguished considering bilayers with a 3R (AB stacking)/2H (AA' stacking) interface as well as bilayers with 2H/2H boundaries. An in situ heating holder is used in ADF-STEM and the GBs are stable to at least 800 °C, with negligible thermally induced reconstructions observed. Normal dislocation cores are seen in one WS2 layer, but the second WS2 layer has different dislocation structures not seen in freestanding monolayers, which have metal-rich clusters to accommodate the stacking mismatch of the 2H:3R interface. These results reveal the competition between maintaining van der Waals bilayer stacking uniformity and dislocation cores required to stitch tilted bilayer GBs together.

Postgrowth Substitutional Tin Doping of 2D WS2 Crystals Using Chemical Vapor Deposition.

Doping of two-dimensional materials provides them tunable physical properties and widens their applications. Here, we demonstrate the postgrowth doping strategy in monolayer and bilayer tungsten disulfide (WS2) crystals, which utilizes a metal exchange mechanism, whereby Sn atoms become substitutional dopants in the W sites by energetically favorable replacement. We achieve this using chemical vapor deposition techniques, where high-quality grown WS2 single crystals are first grown and then subsequently reacted with a SnS precursor. Thermal control of the exchange doping mechanism is revealed, indicating that a sufficiently high enough temperature is required to create the S vacancies that are the initial binding sites for the SnS precursor and metal exchange occurrence. This results in a better control of dopant distribution compared to the tradition all-in-one approach, where dopants are added during the growth phase. The Sn dopants exhibit an n-type doping behavior in the WS2 layers based on the decreased threshold voltage obtained from transistor device measurements. Annular dark-field scanning transmission electron microscopy shows that in bilayer WS2 the Sn doping occurs only in the top layer, creating vertical heterostructures with atomic layer doping precision. This postgrowth modification opens up ways to selectively dope one layer at a time and construct mixed stoichiometry vertical heterojunctions in bilayer crystals.

Atomic structural catalogue of defects and vertical stacking in 2H/3R mixed polytype multilayer WS2 pyramids.

We examine the atomic structure of chemical vapour deposition grown multilayer WS2 pyramids using aberration corrected annular dark field scanning transmission electron microscopy coupled with an in situ heating holder. The stacking orders and specific types of defects after partial degradation by S and W atomic loss at high temperature are resolved layer-by-layer. Our study of an individual WS2 pyramid with at least six layers, reveals a mixed 2H and 3R polytype stacking. Etching occurred both top and bottom of the WS2 pyramid, which aids in determining the exact vertical layer stacking configurations in the thicker regions. We provide an extensive catalogue of the contrast profiles associated with defects in WS2 as a function of layer number and stacking type, as imaged using ADF-STEM. These results provide extensive details about the identification of a wide range of defects in S2 layers, and the unique ADF-STEM contrast patterns that arise from complex multilayer stacking.

High-Performance WS2 Monolayer Light-Emitting Tunneling Devices Using 2D Materials Grown by Chemical Vapor Deposition.

The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel light-emitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.

High Photoresponsivity in Ultrathin 2D Lateral Graphene:WS2:Graphene Photodetectors Using Direct CVD Growth.

We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS2:graphene leads to significant increase in photodetector responsivity. This is achieved by directly growing WS2 using chemical vapor deposition (CVD) in prepatterned graphene gaps to create epitaxial interfaces. Direct-CVD-grown graphene:WS2:graphene lateral photodetecting transistors exhibit high photoresponsivities reaching 121 A/W under 2.7 × 105 mW/cm2 532 nm illumination, which is around 2 orders of magnitude higher than similar devices made by the layer-by-layer transfer method. The photoresponsivity of our direct-CVD-grown device shows negative correlation with illumination power under different gate voltages, which is different from similar devices made by the transfer method. We show that the high photoresponsivity is due to the lowering of effective Schottky barrier height by improving the contact between graphene and WS2. Furthermore, the direct CVD growth reduces overlapping sections of WS2:Gr and leads to more uniform lateral systems. This approach provides insights into scalable manufacturing of high-quality 2D lateral electronic and optoelectronic devices.

Role of Wnt/ß-catenin pathway agonist SKL2001 in Caerulein-induced acute pancreatitis.

The goal of this study was to clarify the protective role of the Wnt/ß-catenin pathway agonist SKL2001 in a rat model of Caerulein-induced acute pancreatitis. AR42J cells and rats were divided into 4 groups: control, Caerulein, SKL2001 + Caerulein, and SKL2001 + control. Cell apoptosis was examined using flow cytometry. Hematoxylin-eosin staining was performed to observe pathological changes in pancreatic and small intestinal tissues. Inflammatory cytokines were detected by enzyme-linked immunosorbent assay (ELISA), while genes related to the Wnt/ß-catenin pathway were quantified using quantitative real-time PCR. In vitro results showed that Caerulein promoted cell necrosis, inhibited the Wnt/ß-catenin pathway, and increased the level of inflammatory cytokines. However, SKL2001 reduced cell necrosis and inflammatory cytokines and activated the Wnt/ß-catenin pathway. Additionally, in vivo results demonstrated the accumulation of fluid (i.e., edema), hemorrhage, inflammation and necrosis of the pancreatic acini occurred 6 h after the final Caerulein induction, with the damage reaching a maximal level 12 h after the final Caerulein induction; meanwhile, the Wnt/ß-catenin pathway was evidently inhibited with an enhanced level of inflammatory cytokines. The aforementioned damage was further aggravated 12 h later. Nevertheless, the pancreatic and small intestinal tissue damages were alleviated in Caerulein-induced rats treated with SKL2001. In conclusion, activation of the Wnt/ß-catenin pathway could inhibit Caerulein-induced cell apoptosis and inflammatory cytokine release, thus improving pancreatic and intestinal damage in rats with acute pancreatitis.

High-Performance Two-Dimensional Schottky Diodes Utilizing Chemical Vapour Deposition-Grown Graphene-MoS2 Heterojunctions.

Heterostructures based on two-dimensional (2D) materials have attracted enormous interest as they display unique functionalities and have potential to be applied in next-generation electronics. In this report, we fabricated three types of heterostructures based on chemical vapor deposition-grown graphene and MoS2. A significant rectification was observed in the Au-MoS2-Gr heterojunction, with a rectification ratio over 2 × 104. The rectifying behavior is reproducible among nearly all 44 devices and is attributed to an asymmetrical Schottky barrier at Au-MoS2 and MoS2-graphene contacts. This rectification can be tuned by external gating and laser illumination, which have different impact on the rectifying ratio. This modulation of the Schottky barrier is evidenced by output characteristics of two symmetrical heterostructures: Au-MoS2-Au and Gr-MoS2-Gr field-effect transistors. The effective heights of MoS2-graphene and MoS2-Au Schottky barriers and their response to back-gate voltage and laser irradiation were extracted from output characteristics of Au-MoS2-Au and Gr-MoS2-Gr field-effect transistors. The tuned Schottky barriers could be explained by the Fermi level change of graphene and MoS2. These results contributed to our understanding of 2D heterostructures and have potential applications in novel electronics and optoelectronics.

Chemical Vapor Deposition Growth of Two-Dimensional Monolayer Gallium Sulfide Crystals Using Hydrogen Reduction of Ga2S3.

Two-dimensional gallium sulfide (GaS) crystals are synthesized by a simple and efficient ambient pressure chemical vapor deposition (CVD) method using a single-source precursor of Ga2S3. The synthesized GaS structures involve triangular monolayer domains and multilayer flakes with thickness of 1 and 15 nm, respectively. Regions of continuous films of GaS are also achieved with about 0.7 cm2 uniform coverage. This is achieved by using hydrogen carrier gas and the horizontally placed SiO2/Si substrates. Electron microscopy and spectroscopic measurements are used to characteristic the CVD-grown materials. This provides important insights into novel approaches for enlarging the domain size of GaS crystals and understanding of the growth mechanism using this precursor system.

High-Performance All 2D-Layered Tin Disulfide: Graphene Photodetecting Transistors with Thickness-Controlled Interface Dynamics.

Tin disulfide crystals with layered two-dimensional (2D) sheets are grown by chemical vapor deposition using a novel precursor approach and integrated into all 2D transistors with graphene (Gr) electrodes. The Gr:SnS2:Gr transistors exhibit excellent photodetector response with high detectivity and photoresponsivity. We show that the response of the all 2D photodetectors depends upon charge trapping at the interface and the Schottky barrier modulation. The thickness-dependent SnS2 measurements in devices reveal a transition from the interface-dominated response for thin crystals to bulklike response for the thicker SnS2 crystals, showing the sensitivity of devices fabricated using layered materials on the number of layers. These results show that SnS2 has photosensing performance when combined with Gr electrodes that is comparable to other 2D transition metal dichalcogenides of MoS2 and WS2.

Effects of surface oxidation of Cu substrates on the growth kinetics of graphene by chemical vapor deposition.

Although the success of graphene research has opened up a new route for wearable electronic and optoelectronic devices, producing graphene with controllable quality and cost-effective growth on a large scale remains challenging due to the lack of understanding about its growth kinetics. Domain boundaries interrupt lattice continuity of graphene; therefore, lowering the nucleation density at the initial stage of graphene growth in the chemical vapor deposition (CVD) process is beneficial for improving the quality of graphene for applications. Herein, we show that by forming an oxide passivation layer on Cu substrates before CVD graphene growth, graphene nucleation density can be effectively decreased. The nucleation mechanism in the presence of an oxide passivation layer is of interest. The analysis of graphene growth kinetics suggests that the thickness of the boundary layer for mass transfer on the substrate surface plays an important role in controlling the reduction rate of the oxide passivation layer. A thick boundary layer created under slow gas flow causes slow reduction of the oxide passivation layer, making finite sites for graphene nucleation. The domain density in a graphene layer is therefore significantly reduced. Graphene sheets of various domain densities (ranging from 104 to 1 mm-2) can be fabricated by suitably choosing the growth parameters. The graphene sheet with a lower density of domain boundaries exhibits better electrical conductivities.

The constituents of roots and stems of Illigera luzonensis and their anti-platelet aggregation effects.

Phytochemical investigation of the roots and stems of Illigera luzonensis afforded two new aporphine alkaloids (1) and (2), one new bisdehydroaporphine alkaloid (3), and one new benzenoid (4), along with 28 known structures. The structures of new compounds were elucidated by spectral and MS analysis. Among the isolated compounds, (1) and (4-13) were subjected into the examination for their inhibitory effects on the aggregation of washed rabbit platelets.

Characterization of cross-linked porous gelatin carriers and their interaction with corneal endothelium: biopolymer concentration effect.

Cell sheet-mediated tissue regeneration is a promising approach for corneal reconstruction. However, the fragility of bioengineered corneal endothelial cell (CEC) monolayers allows us to take advantage of cross-linked porous gelatin hydrogels as cell sheet carriers for intraocular delivery. The aim of this study was to further investigate the effects of biopolymer concentrations (5-15 wt%) on the characteristic and safety of hydrogel discs fabricated by a simple stirring process combined with freeze-drying method. Results of scanning electron microscopy, porosity measurements, and ninhydrin assays showed that, with increasing solid content, the pore size, porosity, and cross-linking index of carbodiimide treated samples significantly decreased from 508±30 to 292±42 µm, 59.8±1.1 to 33.2±1.9%, and 56.2±1.6 to 34.3±1.8%, respectively. The variation in biopolymer concentrations and degrees of cross-linking greatly affects the Young's modulus and swelling ratio of the gelatin carriers. Differential scanning calorimetry measurements and glucose permeation studies indicated that for the samples with a highest solid content, the highest pore wall thickness and the lowest fraction of mobile water may inhibit solute transport. When the biopolymer concentration is in the range of 5-10 wt%, the hydrogels have high freezable water content (0.89-0.93) and concentration of permeated glucose (591.3-615.5 µg/ml). These features are beneficial to the in vitro cultivation of CECs without limiting proliferation and changing expression of ion channel and pump genes such as ATP1A1, VDAC2, and AQP1. In vivo studies by analyzing the rabbit CEC morphology and count also demonstrate that the implanted gelatin discs with the highest solid content may cause unfavorable tissue-material interactions. It is concluded that the characteristics of cross-linked porous gelatin hydrogel carriers and their triggered biological responses are in relation to biopolymer concentration effects.