Improved liquid phase exfoliation technique for the fabrication of MoS2/graphene heterostructure-based photodetector.

2D nanosheets produced using liquid phase exfoliation method offers scalable and cost effective routes to optoelectronics devices. But this technique sometimes yields high defect, low stability, and compromised electronic properties. In this work, we employed an innovative approach that improved the existing liquid phase exfoliation method for fabricating MoS2/graphene heterostructure-based photodetector with enhanced optoelectronic properties. This technique involves hydrothermally treating MoS2 before dispersing it in a carefully chosen and environmentally friendly IPA/water solvent for ultrasonication exfoliation through an optomechanical approach. Thereafter, heterostructure nanosheets of MoS2 and graphene were formed through sequential deposition technique for the fabrication of vertical heterojunctions. Furthermore, we achieved a vertically stacked MoS2/graphene photodetector and a bare MoS2 photodetector. The MoS2/graphene hybrid nanosheets were characterized using spectroscopic and microscopic techniques. The results obtained show the size of the nanosheets is between 350 and 500 nm on average, and their thickness is less than or equal to 5 nm, and high crystallinity in the 2H semiconducting phase. The photocurrent, photoresponsivity, external quantum efficiency (EQE), and specific detectivity of MoS2/graphene heterostructure at 4 V bias voltage and 650 nm illumination wavelength were 3.55 µA, 39.44 mA/W, 7.54 %, and 2.02 × 1010 Jones, respectively, and that of MoS2 photodetector are 0.55 µA, 6.11 mA/W, 1.16 %, and 3.4 × 109 Jones. The results presented indicate that the photoresponse performances of the as-prepared MoS2/graphene were greatly improved (about 7-fold) compared to the photoresponse of the sole MoS2. Again, the MoS2/graphene heterostructure fabricated in this work show better optoelectronic characteristics as compared to the similar heterostructure prepared using the conventional solution processed method. The results provide a modest, inexpensive, and efficient method to fabricate heterojunctions with improved optoelectronic performance.

2D MXene electrochemical transistors.

The solid-state field-effect transistor, FET, and its theories were paramount in the discovery and studies of graphene. In the past two decades another transistor based on conducting polymers, called organic electrochemical transistor (ECT), has been developed and largely studied. The main difference between organic ECTs and FETs is the mode and extent of channel doping; while in FETs the channel only has surface doping through dipoles, the mixed ionic-electronic conductivity of the channel material in organic ECTs enables bulk electrochemical doping. As a result, organic ECTs maximize conductance modulation at the expense of speed. To date ECTs have been based on conducting polymers, but here we show that MXenes, a class of 2D materials beyond graphene, enable the realization of electrochemical transistors (ECTs). We show that the formulas for organic ECTs can be applied to these 2D ECTs and used to extract parameters like mobility. These MXene ECTs have high transconductance values but low on-off ratios. We further show that conductance switching data measured using ECT, in combination with other in situ-ex situ electrochemical measurements, is a powerful tool for correlating the change in conductance to that of the redox state, to our knowledge, this is the first report of this important correlation for MXene films. 2D ECTs can draw great inspiration and theoretical tools from the field of organic ECTs and have the potential to considerably extend the capabilities of transistors beyond those of conducting polymer ECTs, with added properties such as extreme heat resistance, tolerance for solvents, and higher conductivity for both electrons and ions than conducting polymers.

Electrochemically Controlled Hydrogels with Electrotunable Permeability and Uniaxial Actuation.

The unique properties of hydrogels enable the design of life-like soft intelligent systems. However, stimuli-responsive hydrogels still suffer from limited actuation control. Direct electronic control of electronically conductive hydrogels can solve this challenge and allow direct integration with modern electronic systems. An electrochemically controlled nanowire composite hydrogel with high in-plane conductivity that stimulates a uniaxial electrochemical osmotic expansion is demonstrated. This materials system allows precisely controlled shape-morphing at only -1 V, where capacitive charging of the hydrogel bulk leads to a large uniaxial expansion of up to 300%, caused by the ingress of ≈700 water molecules per electron-ion pair. The material retains its state when turned off, which is ideal for electrotunable membranes as the inherent coupling between the expansion and mesoporosity enables electronic control of permeability for adaptive separation, fractionation, and distribution. Used as electrochemical osmotic hydrogel actuators, they achieve an electroactive pressure of up to 0.7 MPa (1.4 MPa vs dry) and a work density of ≈150 kJ m-3 (2 MJ m-3  vs dry). This new materials system paves the way to integrate actuation, sensing, and controlled permeation into advanced soft intelligent systems.

Cost-Effective and Highly Efficient Manganese-Doped MoS2 Nanosheets as Visible-Light-Driven Photocatalysts for Wastewater Treatment.

One of the main objectives in wastewater treatment and sustainable energy production is to find photocatalysts that are favorably efficient and cost-effective. Transition-metal dichalcogenides (TMDs) are promising photocatalytic materials; out of all, MoS2 is extensively studied as a cocatalyst in the TMD library due to its exceptional photocatalytic activity for the degradation of organic dyes due to its distinctive morphology, adequate optical absorption, and rich active sites. However, sulfur ions on the active edges facilitate the catalytic activity of MoS2. On the basal planes, sulfur ions are catalytically inactive. Injecting metal atoms into the MoS2 lattice is a handy approach for triggering the surface of the basal planes and enriching catalytically active sites. Effective band gap engineering, sulfur edges, and improved optical absorption of Mn-doped MoS2 nanostructures are promising for improving their charge separation and photostimulated dye degradation activity. The percentage of dye degradation of MB under visible-light irradiations was found to be 89.87 and 100% for pristine and 20% Mn-doped MoS2 in 150 and 90 min, respectively. However, the degradation of MB dye was increased when the doping concentration in MoS2 increased from 5 to 20%. The kinetic study showed that the first-order kinetic model described the photodegradation mechanism well. After four cycles, the 20% Mn-doped MoS2 catalysts maintained comparable catalytic efficacy, indicating its excellent stability. The results demonstrated that the Mn-doped MoS2 nanostructures exhibit exceptional visible-light-driven photocatalytic activity and could perform well as a catalyst for industrial wastewater treatment.

High-Performance MnO2 Nanowire/MoS2 Nanosheet Composite for a Symmetrical Solid-State Supercapacitor.

To improve the production rate of MoS2 nanosheets as an excellent supercapacitor (SC) material and enhance the performance of the MoS2-based solid-state SC, a liquid phase exfoliation method is used to prepare MoS2 nanosheets on a large scale. Then, the MnO2 nanowire sample is synthesized by a one-step hydrothermal method to make a composite with the as-synthesized MoS2 nanosheets to achieve a better performance of the solid-state SC. The interaction between the MoS2 nanosheets and MnO2 nanowires produces a synergistic effect, resulting in a decent energy storage performance. For practical applications, all-solid-state SC devices are fabricated with different molar ratios of MoS2 nanosheets and MnO2 nanowires. From the experimental results, it can be seen that the synthesized nanocomposite with a 1:4 M ratio of MoS2 nanosheets and MnO2 nanowires exhibits a high Brunauer-Emmett-Teller surface area (∼118 m2/g), optimum pore size distribution, a specific capacitance value of 212 F/g at 0.8 A/g, an energy density of 29.5 W h/kg, and a power density of 1316 W/kg. Besides, cyclic charging-discharging and retention tests manifest significant cycling stability with 84.1% capacitive retention after completing 5000 rapid charge-discharge cycles. It is believed that this unique, symmetric, lightweight, solid-state SC device may help accomplish a scalable approach toward powering forthcoming portable energy storage applications.

Cost-effective synthesis of 2D molybdenum disulfide (MoS2) nanocrystals: An exploration of the influence on cellular uptake, cytotoxicity, and bio-imaging.

Ultrasmall MoS2 nanocrystals have unique optoelectronic and catalytic properties that have acquired significant attraction in many areas. We propose here a simple and economical method for synthesizing the luminescent nanocrystals MoS2 using the hydrothermal technique. In addition, the synthesized MoS2 nanocrystals display photoluminescence that is tunable according to size. MoS2 nanocrystals have many advantages, such as stable dispersion, low toxicity and luminescent characteristics, offering their encouraging applicability in biomedical disciplines. In this study, human lung cancer epithelial cells (A549) are used to assess fluorescence imaging of MoS2 nanocrystals. MTT assay, trypan blue assay, flow cytometry and fluorescence imaging results have shown that MoS2 nanocrystals can selectively target and destroy lung cancer cells, especially drug-resistant cells (A549).

Edge Rich Ultrathin Layered MoS2 Nanostructures for Superior Visible Light Photocatalytic Activity.

Nanostructures of layered 2D materials have been proven one of the significant recent trends for visible-light-driven photocatalysis because of their unique morphology, effective optical adsorption, and rich active sites. Herein, we synthesized ultrathin-layered MoS2 nanoflowers and nanosheets with rich active sites by using a facile hydrothermal technique. The photocatalytic performance of the as-synthesized MoS2 nanoflowers (NF) and nanosheets (NS) were investigated for the photodegradation of MB (methylene blue), MG (malachite Green), and RhB (rhodamine B) dye under visible light irradiations. Ultrathin-layered nanoflowers showed faster degradation (96% in 150 min) in RhB under visible light irradiation, probably due to a large number of active sites and high available surface area. The kinetic study demonstrated that the first-order kinetic model best explained the process of photodegradation. The MoS2 nanoflowers catalysts has similar catalytic performance after four consecutive cyclic performances, demonstrating their good stability. The results showed that the MoS2 nanoflowers have outstanding visible-light-driven photocatalytic activity and could be an effective catalyst for industrial wastewater treatment.

Defects-assisted piezoelectric response in liquid exfoliated MoS2nanosheets.

MoS2is an intrinsic piezoelectric material which offers applications such as energy harvesting, sensors, actuators, flexible electronics, energy storage and more. Surprisingly, there are not any suitable, yet economical methods that can produce quality nanosheets of MoS2in large quantities, hence limiting the possibility of commercialisation of its applications. Here, we demonstrate controlled synthesis of highly crystalline MoS2nanosheets via liquid phase exfoliation of bulk MoS2, following which we report piezoelectric response from the exfoliated nanosheets. The method of piezo force microscopy was employed to explore the piezo response in mono, bi, tri and multilayers of MoS2nanosheets. The effective piezoelectric coefficient of MoS2varies from 9.6 to 25.14 pm V-1. We attribute piezoelectric response in MoS2nanosheets to the defects formed in it during the synthesis procedure. The presence of defects is confirmed by x-ray photoelectron spectroscopy.

Work Function Modulation of Few-Layer Graphene by Swift Heavy Ion Irradiation.

Here, we report the structural and electronic modification induced in chemical vapor deposited graphene by using swift heavy ions (70 MeV Ni6+).Raman spectroscopy was used to quantify the irradiation-induced modification in vibrational properties. The increase in defect density with fluence causes an increase in the intensity ratio of its characteristic Raman D and G band. The increase in defect density also results in a decrease in crystallite size. The changes in the crystal structure are observed from X-rays diffraction measurement. Swift heavy ion irradiation induced defect, modified the surface roughness and surface potential of graphene thin film as measured from atomic force microscopy and scanning Kelvin probe microscopy respectively. The increase in the work function, surface roughness as well as defect concentration with fluence, indicate the possibility of linear correlation between them. Presence of defects in graphene sheets strongly affects surface electronic and optical properties of the material that can be used to tailor the optoelectronics device performance.

Evolution of Raman Spectra in Ion Irradiated MoS2 Atomic Layers: Computational and Experimental Studies.

Herein we report the existence of biaxial strain in swift heavy ion irradiated Molybdenum disulfide (Mos2) as confirmed from Raman spectroscopic measurement and computational study. Defect induced external strain modifies the electronic structure and phonon frequency of the material. In this work, chemically exfoliated Mos2 nanosheets have been exposed to 70 MeV Ni+7 ion irradiation from varying fluence. The Raman spectra reveal that the defect induced LA(M) peak (longitudinal acoustic mode of Phonon at M point) evolves linearly with ion fluence, besides that several other new peaks appear and become visible in Raman spectra thus relaxing Raman fundamental selection rule. Theoretically, simulated Phonon dispersion also supports the fact that tensile strain results in the red shifting of the Raman peak position. The increment of the defect induced LA(M) peak intensity with increasing ion fluence could be a measure of defect quantitatively. This study will be beneficial in the application of external strain to engineer properties of Mos2 as well as understanding the degree of strain inside it quantitatively.

Effect of Ultraviolet Irradiation on Photo-Physical and Surface Electronic Properties of MoS2.

In this study, exfoliation of molybdenum disulfide (MoS2) using the chemical exfoliation method was successfully achieved via probe sonication followed by centrifugation. The observed ultravioletvisible (UV-Vis) spectra of the MoS2 dispersions indicated the presence of a few layers of MoS2. The morphological, structural, optical and surface electronic properties before and after UV light irradiation were investigated by the technique of X-ray diffraction, the Raman spectroscopic measurements, transmission electron microscopy (TEM), and the scanning Kelvin probe microscopy (SKPM). It was observed that after UV irradiation, the Fermi level moves towards the valence band.

Investigation of Swift Heavy Ion Irradiated Reduced Graphene Oxide (rGO)/Molybdenum Disulfide (MoS2) Nanocomposite Using Raman Spectroscopy.

In this work, a few layer molybdenum disulfide (MoS2) and reduced graphene oxide (rGO) nanocomposite have been synthesized by liquid exfoliation method. The morphological and structural properties are analyzed using scanning electron microscopy and X-ray diffraction technique. The optical properties are also investigated using absorption and Raman spectroscopy. This report presents quantification of swift heavy ion irradiation induced defects using Raman spectroscopy. We found both Raman mode E12g and A1g corresponding to MoS2 and Raman modes of rGO are strongly affected by increasing ions doses. The defect induced lattice strain in the rGO/MoS2 nanocomposite is also estimated from Raman spectroscopy. MoS2 layers are found to be much more sensitive than rGO in the rGO/MoS2 nanocomposite. These types of study further used in device based application of rGO/MoS2 nanocomposite system.

Work Function Modulation of Molybdenum Disulfide Nanosheets by Introducing Systematic Lattice Strain.

Tuning the surface electronic properties of 2D transition metal dichalcogenides such as Molebdenum disulfide (MoS2) nanosheets is worth exploring for their potential applications in strain sensitive flexible electronic devices. Here in, the correlation between tensile strain developed in MoS2 nanosheets during swift heavy ion irradiation and corresponding modifications in their surface electronic properties is investigated. With prior structural characterization by transmission electron microscopy, chemically exfoliated MoS2 nanosheets were exposed to 100 MeV Ag ion irradiation at varying fluence for creation of controlled defects. The presence of defect induced systematic tensile strain was verified by Raman spectroscopy and X-ray Diffraction analysis. The effect of ion irradiation on in-plane mode is observed to be significantly higher than that on out-of-plane mode. The contribution of irradiation induced in-plane strain on modification of the surface electronic properties of nanosheets was analyzed by work function measurement using scanning Kelvin probe microscopy. The work function value is observed to be linearly proportional to tensile strain along the basal plane indicating a systematic shifting of Fermi surface with fluence towards the valence band.