Wide spectral photoresponse of layered platinum diselenide-based photodiodes

Platinum diselenide (PtSe2) is a group-10 transition metal dichalcogenide (TMD) that has unique electronic properties, in particular a semimetal-to-semiconductor transition when going from bulk to monolayer form. We report on vertical hybrid Schottky barrier diodes (SBDs) of two-dimensional (2D) PtSe2 thin films on crystalline n-type silicon. The diodes have been fabricated by transferring large-scale layered PtSe2 films, synthesized by thermally assisted conversion of predeposited Pt films at back-end-of-the-line CMOS compatible temperatures, onto SiO2/Si substrates. The diodes exhibit obvious rectifying behavior with a photoresponse under illumination. Spectral response analysis reveals a maximum responsivity of 490 mA/W at photon energies above the Si bandgap and relatively weak responsivity, in the range of 0.1–1.5 mA/W, at photon energies below the Si bandgap. In particular, the photoresponsivity of PtSe2 in infrared allows PtSe2 to be utilized as an absorber of infrared light with tunable sensitivity. The results of our study indicate that PtSe2 is a promising option for the development of infrared absorbers and detectors for optoelectronics applications with low-temperature processing conditions

Exfoliation of 2D materials by high shear mixing

While it has been demonstrated that large scale liquid exfoliation of graphene is possible using high-shear exfoliation, it has not yet been shown to be applicable to a broader range of layered materials. In addition, it would be useful to determine whether the mechanisms reported for shear exfoliation of graphene also apply to other 2D materials. In this work we show that previous models describing high-shear exfoliation of graphene apply to MoS2 and WS2. However, we find the minimum shear rate required to exfoliate MoS2 and WS2 to be ?3 ? 104 s-1, somewhat higher than the value for graphene. We also demonstrate the scalability of shear exfoliation of WS2. By measuring and then optimising the scaling parameters, shear exfoliation of WS2 is shown to be capable of reaching concentrations of 1.82 g l-1 in 6 h and demonstrating a maximum production rate of 0.95 g h-1

Electroanalytical Sensing Properties of Pristine and Functionalized Multilayer Graphene

This paper describes the heterogeneous electron transfer (ET) properties of high-quality multilayer graphene (MLG) films grown using chemical vapor deposition (CVD) on nickel and transferred to insulating poly(ethylene terephthalate) (PET) sheets. An oxygen plasma treatment is used to enhance the ET properties of the films by generating oxygenated functionalities and edge-plane sites and defects. Scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS) along with voltammetry of the standard redox probes [Ru(NH3)6]3+/2+, [Fe(CN)6]3?/4?, and Fe3+/2+ are used to demonstrate this effect. The biologically relevant molecules dopamine, NADH, ascorbic acid, and uric acid are employed to show the improved sensing characteristics of the treated films. Control experiments involving commercially available edge-plane and basal-plane pyrolytic graphite (EPPG and BPPG) electrodes help to explain the different responses observed for each probe, and it is shown that, in certain cases, treated MLG provides a viable alternative to EPPG, hitherto considered to be the ?best-case scenario? in carbon electrochemistry. This is the first comprehensive study of the electroanalytical properties of pristine and functionalized CVD-grown MLG, and it will serve as an important benchmark in the clarification of ET behavior at graphene surfaces, with a view to the development of novel electrochemical sensors

Grain boundary-mediated nanopores in molybdenum disulfide grown by chemical vapor deposition

Molybdenum disulfide (MoS2) is a particularly interesting member of the family of two-dimensional (2D) materials due to its semiconducting and tunable electronic properties. Currently, the most reliable method for obtaining high-quality industrial scale amounts of 2D materials is chemical vapor deposition (CVD), which results in polycrystalline samples. As grain boundaries (GBs) are intrinsic defect lines within CVD-grown 2D materials, their atomic structure is of paramount importance. Here, through atomic-scale analysis of micrometer-long GBs, we show that covalently bound boundaries in 2D MoS2 tend to be decorated by nanopores. Such boundaries occur when differently oriented MoS2 grains merge during growth, whereas the overlap of grains leads to boundaries with bilayer areas. Our results suggest that the nanopore formation is related to stress release in areas with a high concentration of dislocation cores at the grain boundaries, and that the interlayer interaction leads to intrinsic rippling at the overlap regions. This provides insights for the controlled fabrication of large-scale MoS2 samples with desired structural properties for applications

Suppression of the Shear Raman Mode in Defective Bilayer MoS2

We investigate the effects of lattice disorders on the low frequency Raman spectra of bilayer MoS 2. The bilayer MoS 2 was subjected to defect engineering by irradiation with a 30 keV He + ion beam, and the induced morphology change was characterized by transmission electron microscopy. When increasing the ion dose, the shear mode is observed to red-shift, and it is also suppressed sharply compared to other Raman peaks. We use the linear chain model to describe the changes to the Raman spectra. Our observations suggest that the crystallite size and orientation are the dominant factors behind the changes to the Raman spectra

High Areal Capacity Battery Electrodes Enabled by Segregated Nanotube Networks

Increasing the energy storage capability of lithium-ion batteries necessitates maximization of their areal capacity. This requires thick electrodes performing at near-theoretical specific capacity. However, achievable electrode thicknesses are restricted by mechanical instabilities, with high-thickness performance limited by the attainable electrode conductivity. Here we show that forming a segregated network composite of carbon nanotubes with a range of lithium storage materials (e.g. silicon, graphite and metal oxide particles) suppresses mechanical instabilities by toughening the composite, allowing the fabrication of high-performance electrodes with thicknesses of up to 800 ?m. Such composite electrodes display conductivities up to 104 S m-1 and low charge-transfer resistances, allowing fast charge-delivery and enabling near-theoretical specific capacities, even for thick electrodes. The combination of high thickness and specific capacity leads to areal capacities of up to 45 and 30 mAh cm-2 for anodes and cathodes respectively. Combining optimized composite anodes and cathodes yields full-cells with state-of-the-art areal capacities (29 mAh cm-2) and specific/volumetric energies (480 Wh kg-1 and 1600 Wh L-1)

Engineered Materials Platforms for Novel Wave Phenomena (Metamaterials), 11th International Congress on

In this study highly efficient nonradiative energy transfer from semiconductor quantum dots to monolayer MoS 2 with an efficiency of ~99% is demonstrated. Although the energy transfer efficiency is close to unity, there is little enhancement of the MoS 2 photoluminescence intensity. MoS 2 samples of varying layer thickness were electrically contacted and the optoelectronic performance of the devices was studied before and after adding quantum dots in a sensitizing layer. We find photocurrent enhancements as large as ~12 fold for monolayer MoS 2 devices and enhancements as high as ~4 fold for few layer devices with no change in the photocurrent with the MoS 2 devices of bulk-like thicknesses after adding the QDs

Additive-free MXene inks and direct printing of micro-supercapacitors

Direct printing of functional inks is critical for applications in diverse areas including elec-trochemical energy storage, smart electronics and healthcare. However, the available prin-table ink formulations are far from ideal. Either surfactants/additives are typically involved orthe ink concentration is low, which add complexity to the manufacturing and compromisesthe printing resolution. Here, we demonstrate two types of two-dimensional titanium carbide(Ti3C2Tx) MXene inks, aqueous and organic in the absence of any additive or binary-solventsystems, for extrusion printing and inkjet printing, respectively. We show examples of all-MXene-printed structures, such as micro-supercapacitors, conductive tracks and ohmicresistors on untreated plastic and paper substrates, with high printing resolution and spatialuniformity. The volumetric capacitance and energy density of the all-MXene-printed micro-supercapacitors are orders of magnitude greater than existing inkjet/extrusion-printed activematerials. The versatile direct-ink-printing technique highlights the promise of additive-freeMXene inks for scalable fabrication of easy-to-integrate components of printable electronics

High capacity silicon anodes enabled by MXene viscous aqueous ink

The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated thequest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a criticalthickness, solution-processedfilms typically encounter electrical/mechanical problems,limiting the achievable areal capacity and rate performance as a result. Herein, we show thattwo-dimensional titanium carbide or carbonitride nanosheets, known as MXenes, can be usedas a conductive binder for silicon electrodes produced by a simple and scalable slurry-castingtechnique without the need of any other additives. The nanosheets form a continuousmetallic network, enable fast charge transport and provide good mechanical reinforcementfor the thick electrode (up to 450?m). Consequently, very high areal capacity anodes (up to23.3 mAh cm?2) have been demonstrate

Dependence of Photocurrent Enhancements in Hybrid Quantum Dot-MoS2 Devices on Quantum Dot Emission Wavelength

The spectral dependence of nonradiative energy transfer (NRET) from three spectrally different quantum dot (QD) ensembles to monolayer MoS2 is reported. The QDs with peak emission wavelengths of 450, 530, and 630 nm induce large photocurrent enhancements in the hybrid devices with monolayer MoS2 islands grown by chemical vapor deposition (CVD). NRET efficiencies of over 90% are observed for each of the QD-MoS2 hybrids, with 3-fold to 6-fold photocurrent enhancements, depending on the spectral overlap between the QDs and the monolayer MoS2. We find good agreement between the trends obtained from the NRET rate and the spectral overlap function showing evidence for a F?rster-like energy transfer mechanism in these CdSeS/ZnS QD-MoS2 devices