Effects of precursor pre-treatment on the vapor deposition of WS2 monolayers

Toward Monolayered Solar Cells: Luminescence Properties and Light Soaking in TMDs

The optical and photonic characteristics of monolayer transition metal dichalcogenides (TMDs) play a pivotal role in their functionality as solar cell materials, light-emitting diodes (LEDs), and other electro-optical applications. In this study, we reveal the impact of prolonged illumination on the luminescence properties and Raman spectra of monolayered MoS2 and WS2─a process known as "light soaking". We find a light-induced transition from the physisorption to the chemisorption of ambient O2 and H2O molecules. In parallel, we observe the activation and passivation of defect sites in the samples (depending on their initial defect density), which is attributed to the adsorbed ambient molecules and the resulting light-driven interactions with defect sites. Thus, we can control the active defect density of monolayered TMDs and shed light on the fundamental mechanisms underlying their luminescence properties. Therefore, this work clarifies the source of changes to the luminescence properties of TMDs and opens the path toward their integration into advanced applications that may be affected by light soaking, such as solar cells and energy devices.

Nickel and cobalt-based tungstate nanocomposites as promising electrocatalysts in alkaline direct methanol fuel cells

In this work, a non-precious group metal (non-PGM) electrocatalyst based on transition metals is introduced as a promising solution for enhancing the efficiency of direct methanol fuel cell (DMFC). Nickel-cobalt mixed tungstate was prepared using a simple co-precipitation method with different molar ratios of Ni, Co and W. The prepared materials were tested and validated using different characterization techniques. It was observed using SEM that the materials are agglomerated amorphous random circular nanocomposite structures. The electrochemical performance of the prepared electrocatalysts revealed that the best nanocomposite was the one with the Ni : Co : W ratio of 1 : 1 : 1.5 (W1.5). This composite showed a higher current density of 229 mA cm-2 towards methanol oxidation at a scan rate of 50 mV s-1 in 1 M methanol at 0.6 V, with the lowest onset potential of 0.33 V. The obtained results present a new strong non-PGM material for direct methanol electro-oxidation reactions and open new doors for economic and earth-abundant electrocatalysts as an alternative to expensive commercially available catalysts.

Slc11a1 gene polymorphism influences dextran sulfate sodium (DSS)-induced colitis in a murine model of acute inflammation

Ulcerative Colitis (UC) is an inflammatory disease characterized by colonic mucosal lesions associated with an increased risk of carcinogenesis. UC pathogenesis involves environmental and genetic factors. Genetic studies have indicated the association of gene variants coding for the divalent metal ion transporter SLC11A1 protein (formerly NRAMP1) with UC susceptibility in several animal species. Two mouse lines were genetically selected for high (AIRmax) or low (AIRmin) acute inflammatory responses (AIR). AIRmax is susceptible, and AIRmin is resistant to DSS-induced colitis and colon carcinogenesis. Furthermore, AIRmin mice present polymorphism of the Slc11a1 gene. Here we investigated the possible modulating effect of the Slc11a1 R and S variants in DSS-induced colitis by using AIRmin mice homozygous for Slc11a1 R (AIRmin^RR) or S (AIRmin^SS) alleles. We evaluated UC by the disease activity index (DAI), considering weight loss, diarrhea, blood in the anus or feces, cytokines, histopathology, and cell populations in the distal colon epithelium. AIRmin^SS mice have become susceptible to DSS effects, with higher DAI, IL6, G-CSF, and MCP-1 production and morphological and colon histopathological alterations than AIRmin^RR mice. The results point to a role of the Slc11a1 S allele in DSS colitis induction in the genetic background of AIRmin mice.

Steric Hindrance- and Work Function-Promoted High Performance for Electrochemical CO Methanation on Antisite Defects of MoS2 and WS2

CO methanation from electrochemical CO reduction reaction (CORR) is significant for sustainable environment and energy, but electrocatalysts with excellent selectivity and activity are still lacking. Selectivity is sensitive to the structure of active sites, and activity can be tailored by work function. Moreover, intrinsic active sites usually possess relatively high concentration compared to artificial ones. Here, antisite defects Mo_S2 and W_S2 , intrinsic atomic defects of MoS₂ and WS₂ with a transition metal atom substituting a S₂ column, were investigated for CORR by density functional theory calculations. The steric hindrance from the special bowl structure of Mo_S2 and W_S2 ensured good selectivity towards CO methanation. Coordination environment variation of the active sites, the under-coordinated Mo or W atoms, effectively lowered the work function, making Mo_S2 and W_S2 highly active for CO methanation with the required potential of -0.47 and -0.49 V vs. reversible hydrogen electrode, respectively. Moreover, high concentration of active sites and minimal structural deformation during the catalytic process of Mo_S2 and W_S2 enhanced their attraction for future commercial application.

Growth Mechanism of Alternating Defect Domains in Hexagonal WS2 via Inhomogeneous W-Precursor Accumulation

While a hexagonal WS₂ monolayer, grown by chemical vapor deposition, exhibits distinctive patterns in photoluminescence mapping, segmented with alternating S-vacancy (SV) and W-vacancy (WV) domains in a single crystal, the formation mechanism for native alternating defect domains remains unresolved to date. Here, the formation mechanism of alternating defect domains in hexagonal WS₂ via the precursor accumulation model is experimentally elucidated. A triangular WS₂ seed is initially formed, followed by a hexagonal flake. Alternating W-rich (SV) and W-deficient (WV) domains are constructed in hexagonal WS₂ flake, which is confirmed by confocal photoluminescence mapping and secondary ion mass spectroscopy. This is explained by the accumulation or scarcity of W-precursors at the edge of the WS₂ flake. The W-precursors accumulate near the edges of the initial triangular WS₂ seed over time, while they are deficient near the corners of the triangular WS₂ , eventually forming WV domains in the truncated hexagonal domains. The heterogeneous accumulation becomes more prominent in the presence of H₂ gas through desorption of the W-precursors.

Study on the Growth Parameters and the Electrical and Optical Behaviors of 2D Tungsten Disulfide

Transition-metal dichalcogenides (TMDCs) with atomic thickness are promising materials for next-generation electronic and optoelectronic devices. Herein, we report uniform growth of triangular-shaped (∼40 μm) monolayer WS₂ using the atmospheric-pressure chemical vapor deposition (APCVD) technique in a hydrogen-free environment. We have studied the optical and electrical behaviors of as-grown WS₂ samples. The absorption spectrum of monolayer WS₂ shows two intense excitonic absorption peaks, namely, A (∼630 nm) and B (∼530 nm), due to the direct gap transitions at the K point. Photoluminescence (PL) and fluorescence studies reveal that under the exposure of green light, monolayer WS₂ gives very strong red emission at ∼663 nm. This corresponds to the direct band gap and strong excitonic effect in monolayer WS₂. Furthermore, the efficacy of the synthesized WS₂ crystals for electronic devices is also checked by fabricating field-effect transistors (FETs). FET devices exhibit an electron mobility of μ ∼ 6 cm² V^-1 s^-1, current ON/OFF ratio of ∼10⁶, and subthreshold swing (SS) of ∼641 mV decade^-1, which are comparable to those of the exfoliated monolayer WS₂ FETs. These findings suggest that our APCVD-grown WS₂ has the potential to be used for next-generation nanoelectronic and optoelectronic applications.