atmospheric-pressure-fabrication-large-sized-single-layer-rectangular

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...

a-high-throughput-assay-for-prediction-chemical-toxicity-automated

A High-throughput Assay for the Prediction of Chemical Toxicity by Automated Phenotypic Profiling of <em>Caenorhabditis elegans</em>

Applying toxicity testing of chemicals in higher order organisms, such as mice or rats, is time-consuming and expensive, due to their long lifespan and ...

Shenzhen University

Beijing Center for Disease Prevention and Control/Beijing Center of Preventive Medicine Research, China

Capital Medical University, China

Shenzhen university

Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 ± 0.05 eV for single-layer, 2.10 ± 0.05 eV for bilayer and 1.75 ± 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 ± 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.

Bandgap tunability at single-layer molybdenum disulphide grain boundaries.

The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics etc. The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in "van der Waals (vdW) heterostructures" and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe2), bare graphite, and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO-LUMO gap on the electronic screening effects from the substrate. The monolayer WSe2 interlayer provides substantial, but not complete, screening at the organic/inorganic interface. Our results lay a foundation for the exploitation of the complex interfacial properties of hybrid systems based on TMD materials.

Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides.

We demonstrate a method for the preparation of negative-index fibre Bragg gratings (FBGs) using 800 nm femtosecond laser overexposure and thermal regeneration. A positive-index type I-IR FBG was first inscribed in H2-free single-mode fibre using a femtosecond laser directed through a phase mask, and then a highly polarization dependant phase-shifted FBG (P-PSFBG) was fabricated from the type I-IR FBG by overexposure to the femtosecond laser. Subsequently, the P-PSFBG was thermally annealed at 800 °C for 12 hours. Grating regeneration was observed during thermal annealing, and a negative-index FBG was finally obtained with a high reflectivity of 99.22%, an ultra-low insertion loss of 0.08 dB, a blueshift of 0.83 nm in the Bragg wavelength, and an operating temperature of up to 1000 °C for more than 10 hours. Further annealing tests showed that the thermal stability of the negative-index FBG was lower than that of a type II-IR FBG, but much higher than that of a type I-IR FBG. Moreover, the formation of such a negative-index grating may result from thermally regenerated type IIA photosensitivity.

Negative-index gratings formed by femtosecond laser overexposure and thermal regeneration.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication.

Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide.

Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.

Giant photoluminescence enhancement in tungsten-diselenide-gold plasmonic hybrid structures.

Surface-enhanced Raman scattering (SERS) substrates with high activity and stability are desirable for SERS sensing. Here, we report a new single atomic layer graphitic-CN(S-g-CN) and Ag nanoparticles (NPs) hybrid as high-performance SERS substrates. The SERS mechanism of the highly stable S-g-CN/Ag substrates was systematically investigated by a combination of experiments and theoretical calculations. From the results of XPS and Raman spectroscopies, it was found that there was a strong interaction between S-g-CNand Ag NPs, which facilitates the uniform distribution of Ag NPs over the edges and surfaces of S-g-CNnanosheets, and induces a charge transfer from S-g-CNto the oxidizing agent through the silver surface, ultimately protecting Ag NPs from oxidation. Based on the theoretical calculations, we found that the net surface charge of the Ag atoms on the S-g-CN/Ag substrates was positive and the Ag NPs presented high dispersibility, suggesting that the Ag atoms on the S-g-CN/Ag substrates were not likely to be oxidized, thereby ensuring the high stability of the S-g-CN/Ag substrate. An understanding of the stability mechanism in this system can be helpful for developing other effective SERS substrates with long-term stability.

Use of Single-Layer g-CN/Ag Hybrids for Surface-Enhanced Raman Scattering (SERS).

The measurement of electroosmotic flow (EOF) is important in a capillary electrophoresis (CE) experiment in terms of performance optimization and stability improvement. Although several methods exist, there are demanding needs to accurately characterize ultra-low electroosmotic flow rates (EOF rates), such as in coated capillaries used in protein separations. In this work, a new method, called the two-step method, was developed to accurately and rapidly measure EOF rates in a capillary, especially for measuring the ultra-low EOF rates in coated capillaries. In this two-step method, the EOF rates were calculated by measuring the migration time difference of a neutral marker in two consecutive experiments, in which a pressure driven was introduced to accelerate the migration and the DC voltage was reversed to switch the EOF direction. Uncoated capillaries were first characterized by both this two-step method and a conventional method to confirm the validity of this new method. Then this new method was applied in the study of coated capillaries. Results show that this new method is not only fast in speed, but also better in accuracy.

A two-step method for rapid characterization of electroosmotic flows in capillary electrophoresis.

Polymorphism of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS) exhibit fascinating optical and transport properties. Here, we observe a tunable inverted gap (~0.50 eV) and a fundamental gap (~0.10 eV) in quasimetallic monolayer MoS. Using spectral-weight transfer analysis, we find that the inverted gap is attributed to the strong charge-lattice coupling in two-dimensional transition metal dichalcogenides (2D-TMDs). A comprehensive experimental study, supported by theoretical calculations, is conducted to understand the transition of monolayer MoSon gold film from trigonal semiconducting 1H phase to the distorted octahedral quasimetallic 1T' phase. We clarify that electron doping from gold, facilitated by interfacial tensile strain, is the key mechanism leading to its 1H-1T' phase transition, thus resulting in the formation of the inverted gap. Our result shows the importance of charge-lattice coupling to the intrinsic properties of the inverted gap and polymorphism of MoS, thereby unlocking new possibilities for 2D-TMD-based device fabrication.MoSexhibits multiple electronic properties associated with different crystal structures. Here, the authors observe inverted and fundamental gaps through a designed annealing-based strategy, to induce a semiconductor-to-metal phase transition in monolayer-MoSon Au, facilitated by interfacial strain and electron transfer from Au to MoS.

Tunable inverted gap in monolayer quasi-metallic MoSinduced by strong charge-lattice coupling.

Monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) show interesting optical and electrical properties because of their direct bandgap. However, the low absorption of atomically thin TMDs limits their applications. Here, we report enhanced absorption and optoelectronic properties of monolayer molybdenum disulfide (MoS) by using an asymmetric Fabry-Perot cavity. The cavity is based on a hybrid structure of MoS/ hexagonal boron nitride (BN)/Au/SiOrealized through layer-by-layer vertical stacking. Photoluminescence (PL) intensity of monolayer MoSis enhanced over 2 orders of magnitude. Theoretical calculations show that the strong absorption of MoScomes from photonic localization on the top of the microcavity at optimal BN spacer thickness. The n/nMoShomojunction photodiode incorporating this asymmetric Fabry-Perot cavity exhibits excellent current rectifying behavior with an ideality factor of 1 and an ultrasensitive and gate-tunable external photo gain and specific detectivity. Our work offers an effective method to achieve uniform enhanced light absorption by monolayer TMDs, which has promising applications for highly sensitive optoelectronic devices.

Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials.

Recent experimental and theoretical studies have demonstrated that two-dimensional (2D) transition metal dichalcogenide (TMDC) nanoflakes are one of the most promising candidates for non-noblemetal electrocatalysts for hydrogen evolution reaction (HER). However, it is still challenging to optimize their conductivity and enrich active sites for highly efficient electrochemical performance. Herein, we report a chemical vapor deposition (CVD) and thermal annealing two-step strategy to controllably synthesize hybrid electrocatalysts consisting of metallic NbSnanoflake backbones and a highly catalytic active MoSnanocrystalline shell on polished commercial glass carbon (GC). In addition, the amount of MoSin the hybrids can be easily adjusted. We first demonstrate that a small amount of MoSsignificantly promotes the HER activity of 2D NbSnanoflakes, which is in good agreement with the density functional theory (DFT) calculation results. Moreover, the optimized MoS@NbS/GC electrocatalyst displays superior HER activity with overpotential of -164 mV at -10 mA cm, a small Tafel slope of 43.2 mV dec, and prominent electrochemical stability. This study provides a new path for enhancing the HER performance of 2D TMDC nanoflakes.

MoS-coated NbSnanoflakes grown on glass carbon: an advanced electrocatalyst for the hydrogen evolution reaction.

Tungsten disulfide monolayers have attracted extensive attention in nanoelectronics and optoelectronics applications due to their remarkable electronic and optical properties. High-quality WS2 monolayers with a scalable size and uniform thickness can be synthesized by a chemical vapor deposition method (CVD). However, they commonly contain intrinsic structural defects and different populations of charge carriers, which are responsible for different contributions of excitons, trions, and biexcitons to their photoluminescence (PL) emission. Here, we adopt sodium sulphide (Na2S) solution to chemically treat CVD-grown WS2 monolayers by a simple immersing method. The results show that WS2 monolayers have a significantly enhanced PL emission by a factor of 25-fold and an obvious red-shift of the PL wavelength, resulting from the different excitonic states induced by effective n-type doping after Na2S treatment. This work provides a simple but promising chemical doping route to significantly improve the optical properties of WS2 monolayers and paves the way for the realization of practical WS2 monolayer based optoelectronic applications.

ArticleTotal : 2	Year
Publication title Cited by 3	2018
Publication title Cited by 10	2019

Article	Year
Bandgap tunability at single-layer molybdenum disulphide grain boundaries. Nature communications\| PubMed ID: 25687991	2015
Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides. ACS nano\| PubMed ID: 26792247	2016
Negative-index gratings formed by femtosecond laser overexposure and thermal regeneration. Scientific reports\| PubMed ID: 26979090	2016
Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide. Nano letters\| PubMed ID: 27140667	2016
Giant photoluminescence enhancement in tungsten-diselenide-gold plasmonic hybrid structures. Nature communications\| PubMed ID: 27150276	2016
Use of Single-Layer g-CN/Ag Hybrids for Surface-Enhanced Raman Scattering (SERS). Scientific reports\| PubMed ID: 27687573	2016
A two-step method for rapid characterization of electroosmotic flows in capillary electrophoresis. Electrophoresis\| PubMed ID: 28869669	2017
Tunable inverted gap in monolayer quasi-metallic MoSinduced by strong charge-lattice coupling. Nature communications\| PubMed ID: 28883392	2017
Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials. Nano letters\| PubMed ID: 29115838	2017
MoS-coated NbSnanoflakes grown on glass carbon: an advanced electrocatalyst for the hydrogen evolution reaction. Nanoscale\| PubMed ID: 29393949	2018
Significant photoluminescence enhancement in WSmonolayers through NaS treatment. Nanoscale\| PubMed ID: 29546899	2018

Wenjing Zhang

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