Thickness dependant characterization of chemically exfoliated TiS2 nanosheets

Monolayer TiS2 is the lightest member of the transition metal dichalcogenides family with promising application in energy storage and conversion systems. Use of TiS2 has been limited by the lack of rapid characterisation of layer number via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate layer number dependent Raman modes for TiS2. 1T-TiS2 presents two characteristics Raman active modes, A1g (out-of-plane) and Eg (in-plane). We identified a characteristic peak frequency shift of the Eg mode with the layer number and an unexplored Raman mode at 372 cm-1 whose intensity changes relative to the A1g mode with the thickness of TiS2 sheets. These two characteristic features of the Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS2. Further, we develop a method to produce oxidation-resistant inks of micron sized mono- and few-layered TiS2 nanosheets at concentrations up to 1 mg/mL .These TiS2 inks can be deposited to form thin films with controllable thickness and nanosheet density over cm2 areas. This opens up pathways for a wider utilization of exofliated TiS2 towards a range of applications

Experimental signature of a topological quantum dot

Topological insulators (TIs) present a neoteric class of materials, which support delocalised, conducting surface states despite an insulating bulk. Due to their intriguing electronic properties, their optical properties have received relatively less attention. Even less well studied is their behaviour in the nanoregime, with most studies thus far focusing on bulk samples - in part due to the technical challenges of synthesizing TI nanostructures. We study topological insulator nanoparticles (TINPs), for which quantum effects dominate the behaviour of the surface states and quantum confinement results in a discretized Dirac cone, whose energy levels can be tuned with the nanoparticle size. The presence of these discretized energy levels in turn leads to a new electron-mediated phonon-light coupling in the THz range. We present the experimental realisation of Bi$_2$Te$_3$ TINPs and strong evidence of this new quantum phenomenon, remarkably observed at room temperature. This system can be considered a topological quantum dot, with applications to room temperature THz quantum optics and quantum information technologies

Ступінь приверженості до лікування та його ефективність у пацієнтів з гіпертонічною хворобою залежно від способу життя

Vitamin C is a widely used vitamin. Here we review the occurrence and properties of aldonolactone oxidoreductases, an important group of flavoenzymes responsible for the ultimate production of vitamin C and its analogs in animals, plants, and single-cell organisms

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T′) phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T′) phase. Bottom-up synthesis of materials in the 1T(1T′) phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T′) phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control

Direct solution-phase synthesis of 1T’ WSe2 nanosheets

Crystal phase control in layered transition metal dichalcogenides is central for exploiting their different electronic properties. Access to metastable crystal phases is limited as their direct synthesis is challenging, restricting the spectrum of reachable materials. Here, we demonstrate the solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T’ phase) of WSe2 nanosheets. We design a kinetically-controlled regime of colloidal synthesis to enable the formation of the metastable phase. 1T’ WSe2 branched few-layered nanosheets are produced in high yield and in a reproducible and controlled manner. The 1T’ phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400 °C. The 1T’ WSe2 nanosheets demonstrate a metallic nature exhibited by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe2 nanosheets and comparable to other 1T’ phases. This synthesis design can potentially be extended to different materials providing direct access of metastable phases

Genetic diversity and relatedness within and between species of the genus Oligoryzomys (Rodentia; Sigmodontinae)

A RAPD analysis on six species of the rodent genus Oligoryzomys trapped in a wide area (ranging from 01° N to 32° S) of Brazilian territory was performed in order to determine the levels of genetic variability within and between its populations and species. One-hundred and ninety-three animals were collected in 13 different sites (corresponding to 17 samples) located at Pampas, Atlantic Rain Forest, Cerrado, and Amazon domains. Oligoryzomys sp., O. nigripes (8 populations), O. flavescens (4 populations), O. moojeni, O. stramineus, and O. fornesi were the taxa analyzed. Of the 20 primers tested, 4 generated a total of 75 polymorphic products simultaneously amplified in 151 specimens. Various diversity estimators analyzed showed considerable differences between species and populations, indicating a great genetic variation occurring in the Oligoryzomys taxa investigated. A cluster analysis was made using Nei's standard genetic distances, however, it did not correlate the genetic heterogeneity of the species and populations with the geographical areas

When epitaxy meets plasma: a path to ordered nanosheets arrays

The possibility of a controlled assembly of 2-dimensional (2D) nanosheets (NSs) into ordered arrays or even more sophisticated structures offers tremendous opportunities in the context of fabrication of a variety of NSs based devices. Reports of such ordered NSs are rare and all conventional “top-down” methods typically led to coarse structures exhibiting only limited surface quality. In this work, we demonstrate a path to directly synthesis ordered NSs arrays in a plasma activated chemical vapor deposition technique utilizing planar defects formed during hetero-epitaxial growth of crystals featuring a close-packed lattice. As an example, the synthesis of 3C-SiC NSs arrays with well-defined orientation on (001) and (111) Si substrates is shown. A detailed analysis identifies planar defects and the plasma environment as key factors determining the resulting 2D NSs arrays. Consequently, a “planar defects induced selective growth” effect is proposed to elucidate the corresponding growth mechanism

Large-area CVD MoS2/WS2 heterojunctions as a photoelectrocatalyst for salt water oxidation

Splitting salt water via sunlight into molecular oxygen and hydrogen for use as fuel or as an energy carrier is a clear pathway toward renewable energy. Monolayer MoS2 and WS2 are promising materials for the energetically demanding water oxidation reaction, absorbing ∼10% of incident light in the visible spectrum and possessing chemical stability and band edges more positive than the oxidation potential of water. A heterostructure of MoS2/WS2 forms a type-II heterojunction, supporting fast separation of the photogenerated charge carriers across the junction. Here, we show the role played by defects in determining the efficiency of the photon-driven oxidation process. By reducing the defects in this material system, it is possible to obtain an incident photon-to-current conversion efficiency (IPCE) of ∼1.6% and a visible-light-driven photocurrent density of 1.7 mA/cm2 for water oxidation. The efficiency is one order of magnitude higher than that of photoelectrocatalytic hydrogen reduction and water oxidation supported by liquid-phase exfoliated transition-metal dichalcogenides (TMDs). This result has been achieved with chemically vapor deposited (CVD) MoS2/WS2 heterojunctions, in the form of 100 μm large flakes assembled to form thin films. The large flakes sizes, compared to liquid-phase exfoliated materials (normally <5 μm), and thus the low edge flake density, and the flakes’ atomically sharp and clean interfaces between the flakes are responsible for reducing charge carrier recombination. These results show a general approach to the scalable synthesis of high-crystal-quality low-dimensional semiconductor photoelectrodes for solar energy conversion systems. It also shows the uniqueness of the CVD synthesis process of these materials, which can lead to high quality materials without the need of any postsynthesis treatments

MoS2/WS2 heterojunction for photoelectrochemical water oxidation

The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS2 and WS2 can oxidize water to O2 under incident light. Thin films of solution-processed MoS2 and WS2 nanosheets display n-type positive photocurrent densities of 0.45 mA cm–2 and O2 evolution under simulated solar irradiation. WS2 is significantly more efficient than MoS2; however, bulk heterojunctions (B-HJs) of MoS2 and WS2 nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS2 and WS2 nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production