Engineering Catalytically Active Sites by Sculpting Artificial Edges on MoS2 Basal Plane for Dinitrogen Reduction at a Low Overpotential.

Engineering catalytically active sites have been a challenge so far and often relies on optimization of synthesis routes, which can at most provide quantitative enhancement of active facets, however, cannot provide control over choosing orientation, geometry and spatial distribution of the active sites. Artificially sculpting catalytically active sites via laser-etching technique can provide a new prospect in this field and offer a new species of nanocatalyst for achieving superior selectivity and attaining maximum yield via absolute control over defining their location and geometry of every active site at a nanoscale precision. In this work, a controlled protocol of artificial surface engineering is shown by focused laser irradiation on pristine MoS2 flakes, which are confirmed as catalytic sites by electrodeposition of AuNPs. The preferential Au deposited catalytic sites are found to be electrochemically active for nitrogen adsorption and its subsequent reduction due to the S-vacancies rather than Mo-vacancy, as advocated by DFT analysis. The catalytic performance of Au-NR/MoS2 shows a high yield rate of ammonia (11.43 × 10-8  mol s-1 cm-2 ) at a potential as low as -0.1 V versus RHE and a notable Faradaic efficiency of 13.79% during the electrochemical nitrogen reduction in 0.1 m HCl.

Sculpting Artificial Edges in Monolayer MoS2 for Controlled Formation of Surface-Enhanced Raman Hotspots.

Hotspot engineering has the potential to transform the field of surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive and reproducible detection of analytes. However, the ability to controllably generate SERS hotspots, with desired location and geometry, over large-area substrates, has remained elusive. In this study, we sculpt artificial edges in monolayer molybdenum disulfide (MoS2) by low-power focused laser-cutting. We find that when gold nanoparticles (AuNPs) are deposited on MoS2 by drop-casting, the AuNPs tend to accumulate predominantly along the artificial edges. First-principles density functional theory (DFT) calculations indicate strong binding of AuNPs with the artificial edges due to dangling bonds that are ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation of AuNPs along the artificial edges intensifies plasmonic effects in these regions, creating hotspots exclusively along the artificial edges. DFT further indicates that adsorption of AuNPs along the artificial edges prompts a transition from semiconducting to metallic behavior, which can further intensify the plasmonic effect along the artificial edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2 surface (away from the cut edges) or along the natural (passivated) edges of the MoS2 sheet. To demonstrate the practical utility of this concept, we use our substrate to detect Rhodamine B (RhB) with a large SERS enhancement (∼104) at the hotspots for RhB concentrations as low as ∼10-10 M. The single-step laser-etching process reported here can be used to controllably generate arrays of SERS hotspots. As such, this concept offers several advantages over previously reported SERS substrates that rely on electrochemical deposition, e-beam lithography, nanoimprinting, or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety of 2D material platforms.

Controlled nanofabrication of metal-free SERS substrate on few layered black phosphorus by low power focused laser irradiation.

Black Phosphorous (BP) has intrinsic in-plane ferroelectric properties that may have the inherent capability of SERS response and can be considered as a replacement of metal nanoparticle-based SERS substrates. A simple one-step process has been demonstrated for the controlled nano-structuring and rapid prototyping on a BP flake to develop a metal-free SERS substrate by low power focused laser irradiation. The effect of focused laser irradiation on the surface morphology of the pristine BP flakes has been thoroughly investigated by real time Raman spectroscopy measurements and corresponding AFM height profiling, which confirms that the proposed laser irradiation technique has more advantages over the conventional lithography and is free from undesired contamination. For a 532 nm laser line, the minimum laser power needed to create a nano-void on the BP flake is 25 mW (Power density = ∼15.62 × 105 W cm-2) with 5 s exposure time, where the etching rate is controlled by the laser power and exposure time. By analyzing the geometrical shape of the nano-void created due to laser irradiation, it is possible to identify the armchair and zigzag directions of the BP flake. The experimental results revealed that by controlling the exposure time and laser power, it was possible to perform layer by layer thinning of BP flakes. The proposed thinning process of the BP flake did not alter the pristine quality and no signature of oxidation was found in the Raman spectra, which signified the reliability of this low power laser irradiation technique towards the future nano-fabrication of BP-based devices. The controlled formation of the nano-void array on a few layered BP flake enhanced the local electric field (hot spots) in the vicinity of the nano-voids, resulting in ∼30% Raman intensity enhancement. Such nano-void induced hotspots on the BP flake open up a new species of metal-free SERS substrate, demonstrating pronounced enhancement in the Raman signal of Rhodamine B as high as ∼106 and a limit of detection (LOD) up to ∼10 nM.

Ultrafast Carrier Dynamics of the Exciton and Trion in MoS2 Monolayers Followed by Dissociation Dynamics in Au@MoS2 2D Heterointerfaces.

Many-body states like excitons, biexcitons, and trions play an important role in optoelectronic and photovoltaic applications in 2D materials. Herein, we studied carrier dynamics of excitons and trions in monolayer MoS2 deposited on a SiO2/Si substrate, before and after Au NP deposition, using femtosecond transient absorption spectroscopy. Luminescence measurements confirm the presence of both an exciton and trion in MoS2, which are drastically quenched after deposition of Au NPs, indicating electron transfer from photoexcited MoS2 to Au. Ultrafast study reveals that photogenerated free carriers form excitons with a time scale of ∼500 fs and eventually turn into trions within ∼1.2 ps. Dissociation of excitons and trions has been observed in the presence of Au, with time scales of ∼600 fs and ∼3.7 ps, respectively. Understanding the formation and dissociation dynamics of the exciton and trion in monolayer MoS2 is going to help immensely to design and develop many new 2D devices.

Modulating capacitive response of MoS2 flake by controlled nanostructuring through focused laser irradiation.

Unlike graphene nanostructures, various physical properties of nanostructured MoS2 have remained unexplored due to the lack of established fabrication routes. Herein, we have reported unique electrostatic properties of MoS2 nanostructures, fabricated in a controlled manner of different geometries on 2D flake by using focused laser irradiation technique. Electrostatic force microscopy has been carried out on MoS2 nanostructures by varying tip bias voltage and lift height. The analysis depicts no contrast flip in phase image of the patterned nanostructure due to the absence of free surface charges. However, prominent change in phase shift at the patterned area is observed. Such contrast changes signify the capacitive interaction between tip and nanostructures at varying tip bias voltage and lift height, irrespective of their shape and size. Such unperturbed capacitive behavior of the MoS2 nanostructures offer modulation of capacitance in periodic array on 2D MoS2 flake for potential application in capacitive devices.

Graphene Oxide Demonstrates Experimental Confirmation of Abraham Pressure on Solid Surface.

The century-old controversy over two contradicting theories on radiation pressure of light proposed by Abraham and Minkowski can come to an end if there is a direct method to measure the surface deformation of the target material due to momentum transfer of photons. Here we have investigated the effect of radiation pressure on the surface morphology of Graphene Oxide (GO) film, experienced due to low power focused laser irradiation. In-depth investigation has been carried out to probe the bending of the GO surface due to radiation pressure by Atomic Force Microscopy (AFM) and subsequently the uniaxial strain induced on the GO film has been probed by Raman Spectroscopy. Our results show GO film experience an inward pressure due to laser radiation resulting in inward bending of the surface, which is consistent with the Abraham theory. The bending diameter and depth of the irradiated spot show linear dependence with the laser power while an abrupt change in depth and diameter of the irradiated spot is observed at the breaking point. Such abrupt change in depth is attributed to the thinning of the GO film by laser irradiation.

Goiter prevalence in school-going children: A cross-sectional study in two border districts of sub-Himalayan Jammu and Kashmir.

INTRODUCTION: Iodine deficiency disorder remains one of the major public health problems, despite it being easily preventable. Initially thought as the disorder of Himalayan goiter belt, the disorder has been found to have a pan-India presence. The study was planned with the aim to assess the prevalence of goiter among 6-12 years school-going children and to identify levels of iodine in salt consumed at the household level in the study area. MATERIALS AND METHODS: The study was conducted among 3955 school-going (2162 children from Rajouri and 1793 children from Poonch) children age 6-12 years in Rajouri and Poonch districts of Jammu Province. The children were examined clinically to detect and grade the goiter. Salt samples were collected from subsample (n = 400) to estimate iodine content in the salt. RESULTS: The prevalence of goiter in the study population was found to be 18.87% and 9.70% in Rajouri and Poonch districts, respectively. Goiter prevalence was higher in 9-12 years age group as compared to 6-9 years age group and the difference in goiter prevalence in males of these two age groups was statistically significant (P = 0.02). Estimation of iodine content of the salt samples revealed that all of them (100%) had adequate iodine content. CONCLUSION: Despite the implementation of Universal Iodization Program and adequate content of iodine in salt consumed by the families, the goiter prevalence in both the districts is high. The future research should focus on identifying the reasons for this high prevalence.

Development of biosensors for the detection of biological warfare agents: its issues and challenges.

This review discusses current development in biosensors for the detection of biological warfare agents (BWAs). BWAs include bacteria, virus and toxins that are added deliberately into air water and food to spread terrorism and cause disease or death. The rapid and unambiguous detection and identification of BWAs with early warning signals for detecting possible biological attack is a major challenge for government agencies particularly military and health. The detection devices--biosensors--can be classified (according to their physicochemical transducers) into four types: electrochemical, nucleic acid, optical and piezoelectric. Advantages and limitations of biosensors are discussed in this review followed by an assessment of the current state of development of different types of biosensors. The research and development in biosensors for biological warfare agent detection is of great interest for the public as well as for governments.