High-resolution Raman Spectroscopy Research Articles

Tailored eutectic alloy coating for enhanced EMI and X-ray protection by basalt fiber CNT/epoxy composite

In this study, eutectic-Bi–Sn-coated basalt fiber (BF)-reinforced carbon nanotube (CNT)/epoxy hybrid composites were fabricated for dual functionality, i.e., in managing and shielding clinical X-ray radiation and electromagnetic interference (EMI). BF mats were coated with Bi–Sn nanoparticles and subsequently layered with multi-walled CNTs mixed epoxy resin using the vacuum-assisted resin transfer method. High resolution field emission scanning electron microscopy revealed a good dispersion of Bi–Sn nanoparticles over BF and, CNTs within the epoxy matrix. X-ray diffraction analysis confirmed the presence of Bi–Sn, basalt, and CNT phases in the composites. High-resolution Raman spectroscopy revealed characteristic peaks corresponding to the CNTs, epoxy, and Bi–Sn phases. EMI total shielding effectiveness (SET) analysis in the X-band frequency range (8.2–12.4 GHz) demonstrated that the Bi–Sn/BF/CNT/epoxy (S3) exhibits the highest SET value of 30.4 dB, which is attributable to the synergistic effect of the Bi–Sn coating and CNT filler. Analysis of X-ray-radiation leakage revealed that all the composite samples effectively attenuated X-rays with minimal leakage, limited to 6.8 mR. These results indicate the potential of these composites for applications as eco-friendly, non-toxic, and lead-free various industries including healthcare, electronics, aerospace, and defense.

Mango Leaves (Mangifera indica)-Derived Highly Florescent Green Graphene Quantum Dot Nanoprobes for Enhanced On-Off Dual Detection of Cholesterol and Fe2+ Ions Based on Molecular Logic Operation.

In the present study, we have engineered a molecular logic gate system employing both Fe2+ ions and cholesterol as bioanalytes for innovative detection strategies. We utilized a green-synthesis method employing the mango leaves extract to create fluorescent graphene quantum dots termed "mGQDs". Through techniques like HR-TEM, i.e., high-resolution transmission electron microscopy, Raman spectroscopy, and XPS, i.e., X-ray photoelectron spectroscopy, the successful formation of mGQDs was confirmed. The photoluminescence (PL) characteristics of mGQDs were investigated for potential applications in metal ion detection, specifically Fe2+ traces in water, by using fluorescence techniques. Under 425 nm excitation, mGQDs exhibited emission bands at 495 and 677 nm in their PL spectrum. Fe2+-induced notable quenching of mGQDs' PL intensity decreased by 97% with 2.5 μM Fe2+ ions; however, adding 20 mM cholesterol resulted in a 92% recovery. Detection limits were established through a linear Stern-Volmer (S-V) plot at room temperature, yielding values of 4.07 μM for Fe2+ ions and 1.8 mM for cholesterol. Moreover, mGQDs demonstrated biocompatibility, aqueous solubility, and nontoxicity, facilitating the creation of a rapid nonenzymatic cholesterol detection method. Selectivity and detection studies underscored mGQDs' reliability in cholesterol level monitoring. Additionally, a molecular logic gate system employing Fe2+ metal ions and cholesterol as a bioanalyte was established for detection purposes. Overall, this research introduces an ecofriendly approach to craft mGQDs and highlights their effectiveness in detecting metal ions and cholesterol, suggesting their potential as versatile nanomaterials for diverse analytical and biomedical applications.

Large-Area Synthesis and Fabrication of Few-Layer hBN/Monolayer RGO Heterostructures for Enhanced Contact Surface Potential.

Hexagonal boron nitride (hBN) has a property similar to that of graphene, and it has become one of the most popular materials due to its flexible physical and chemical properties for a variety of applications, especially in nanoelectronics. Enhanced properties of hBN-based heterostructures are crucial for future electronic devices. In this work, a sheet-like hBN crystal was synthesized and transferred onto SiO2/Si substrate and reduced graphene oxide (RGO)/SiO2/Si substrate. Accordingly, the hBN and hBN/RGO films are investigated by optical microscopy, X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The thickness of a single hBN layer is approximately 0.4 nm. A few layers of hBN stacked in large areas are mostly observed in both hBN and the hBN/RGO films. By using Kelvin probe force microscopy, it was found that the hBN/RGO heterostructure has a contact surface potential higher than that of the hBN layer. The large-scale synthesis and fabrication of hBN/RGO films could be extended to fabricate other van der Waals heterostructures.

Competitive nature of weak anti-localization and weak localization effect in Cr-doped sputtered topological insulator Bi2Se3 thin film

In the present study, we have investigated the effect of magnetic Cr doping on the transport properties of a sputtered Bi2Se3 thin film on the SrTiO3 (110) substrate. The high-resolution x-ray diffraction and Raman spectroscopy measurements revealed the growth of rhombohedral Bi2Se3 thin films. Further electronic and compositional analysis was done by x-ray photoemission spectroscopy and Rutherford backscattering spectroscopy, and the x-value was estimated to be 0.18 in the Bi2−xCrxSe3 thin film. The variation in the resistivity with temperature (2–300 K) revealed the metallic nature in undoped Bi2Se3 up to 30 K and upturn resistivity below 30 K. The Cr-doped Bi2Se3 resistivity data show a traditional semiconducting nature up to 25 K and take an abrupt upturn resistivity below 25 K. The resistivity behavior of both samples was explained by adopting a model that consists of the total resistance, a combination of bulk and surface resistance in parallel. The bulk bandgap value determined by this method is obtained to be 256 meV in an undoped Bi2Se3 thin film. Magnetoconductance data of the undoped thin film revealed a weak anti-localization (WAL) effect, while the Cr-doped thin film showed a weak localization (WL) effect at low temperatures (<50 K). At low magnetic field and low temperature, a competing nature of WAL and WL effects was prominent in the Cr-doped film. A drastic increase in the electrical resistance suggests that Cr doping can significantly modify the electrical properties of Bi2Se3 thin films, which could have potential applications in futuristic devices.

Synergistic effect of graphene and nanodiamonds to achieve ultra-low friction on rough DLC coatings

Diamond-like carbon (DLC) with its protective, antiwear and self-lubricant properties represents an outstanding coating material to improve the tribological performance of bare metal-to-metal contacts. The DLC friction response is strictly related to the generation of a carbon-rich transfer layer on the sliding countersurfaces. This key process may benefit from the DLC surface functionalization with carbon nanomaterials, as attested by the recent observation of wearless sliding and ultra-low friction for nanorough model interfaces. In this study, we show by ball-on-disc tribometer that the tribological response of microrough steel-DLC contacts is considerably improved in dry atmosphere through DLC functionalization with graphene sheets (GSs) and nanodiamonds (NDs). The functionalization effectiveness is demonstrated exclusively with both allotropic forms, that synergically lower friction after a unique run-in period. We attribute the enhanced lubricity, with steady coefficient of friction <0.05 at 1 N load, to the formation of a transfer layer which incorporates GSs, NDs and nanoscroll structures self-assembled during sliding. High-resolution electron microscopy and Raman spectroscopy indicate that NDs, besides enabling the erosion of the highest interfacial micro-asperities, do assist the transfer layer development via GSs milling. Our findings contribute to the current quest for superlubricity in realistic tribocontacts meeting industrial standards.

Effects of ammonia addition on the soot nanostructure and oxidation reactivity in n-heptane/toluene diffusion flames

Co-firing NH3 with conventional hydrocarbon fuels is an important approach for reducing CO2 emissions in existing combustion systems. Besides CO2, the blending of NH3 would also notably affect soot formation and its oxidation behaviors. In the present study, we focus on the effects of NH3 on the nanostructure and oxidation characteristics of soot produced in diffusion flames of n-heptane/toluene mixtures. Two configurations of laminar co-flow diffusion flame, including both normal and inverse diffusion flames (NDF and IDFs), were used for investigation. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (Raman), and Thermogravimetric analysis (TGA) were employed for soot characterization. The HRTEM and Raman spectra showed that with the increase of NH3 blending ratio, the fringe length (La) and the degree of graphitization decreased while the microcrystal tortuosity (Tf) increased. The results are in consistent with TGA analysis which suggests the promoting effects of NH3 on the soot oxidation reactivity. Difference between NDF and IDF with respect to the soot nanostructure and oxidation activity were discussed. It is our hope that the present results could deepen our understanding on the effects of NH3 on soot nanostructure and oxidation behavior and benefit the design of particulate filters for combustion devices fueled with hydrocarbon/NH3 mixtures.

Large area MoSe2 and MoSe2/Bi2Se3 films on sapphire (0001) for near-infrared photodetection

The fabrication of heterojunction-based photodetectors (PDs) is well known for the enhancement of PDs performances, tunable nature of photoconductivity, and broadband application. Herein, the PDs based on MoSe2 and MoSe2/Bi2Se3 heterojunction on sapphire (0001) substrates were deposited using a r.f. magnetron sputtering system. The high-resolution x-ray diffraction and Raman spectroscopy characterizations disclosed the growth of the 2-H phase of MoSe2 and the rhombohedral phase of Bi2Se3 thin films on sapphire (0001). The chemical and electronic states of deposited films were studied using x-ray photoelectron spectroscopy and revealed the stoichiometry growth of MoSe2. We have fabricated metal-semiconductor–metal type PD devices on MoSe2 and MoSe2/Bi2Se3 heterojunction and the photo-response measurements were performed at external voltages of 0.1–5 V under near-infrared (1064 nm) light illumination. The bare MoSe2 PD device shows positive photoconductivity behavior whereas MoSe2/Bi2Se3 heterojunction PD exhibits negative photoconductivity. It was found that the responsivity of MoSe2 and MoSe2/Bi2Se3 heterojunction PDs is ~ 1.39 A W−1 and ~ 5.7 A W−1, respectively. The enhancement of photoresponse of MoSe2/Bi2Se3 PD nearly four-fold compared to bare MoSe2 PD shows the importance of heterojunction structures for futuristics optoelectronic applications.

High-pressure study of synthesized α-Bi2O3/NiBi3 nanostructured composite: In-situ synchrotron XRD measurements, DFT calculations and PDF approach

We investigated a nanostructured composite of α-Bi2O3/NiBi3, which was synthesized through high-energy milling, under high-pressure conditions of up to 30 GPa. To track its structural changes, we employed in-situ synchrotron angle-dispersive X-ray diffraction measurements in conjunction with density functional theory calculations. Crystallographic information was derived using the Rietveld method and DFT computations. Additionally, we examined the chemical short-range order using pair distribution functions and determined the compressibility parameters through the Birch-Murnaghan equation of state.We explored the high-pressure behavior of a nanostructured composite of α-Bi2O3/NiBi3, applying pressures up to 30 GPa. The composite sample, synthesized by mechanical alloying, underwent thorough structural characterization by X-ray diffraction, high-resolution transmission electron microscopy and Raman spectroscopy. The sample's response to applied pressure was studied by analyzing crystallographic data obtained from in-situ synchrotron angle-dispersive X-ray diffraction measurements and density functional theory calculations. Crystallographic information from experiments was refined using the Rietveld method. Additionally, chemical short-range order was examined using pair distribution functions, and compressibility parameters were determined via the Birch-Murnaghan equation of state. This investigation into the high-pressure behavior of NiBi3 provides valuable insights for future studies and potential applications of similar materials in extreme pressure environments.

Novel LiFePO4/very-few-layer-graphene (LFP/VFLG) composites to improve structural and electrochemical properties of lithium-ion battery cathode

LiFePO4/Very-few-layer-graphene (LFP/VFLG) composites have been prepared using the sol-gel method for lithium-ion battery cathode. VFLG dominated by 1∼3 layers was obtained from an economical and environmentally benign process. Structural properties of LFP/VFLG composites were studied using Fourier-transform infrared spectra (FTIR) spectroscopy, X-ray diffractometry (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy, while the electrochemical performance was measured using galvanostatic discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests. The FTIR analysis confirmed that incorporating VFLG neither affected the chemical structure of LiFePO4 nor initiated any side reactions during the LiFePO4 formation reaction. The XRD results showed that LFP/VFLG composites had better lattice parameters, crystallinity, and phase purity than LiFePO4 without VFLG addition. The Raman spectroscopy analysis indicated that LFP/VFLG composites had a lower disorder in the carbon arrangement. HRTEM results indicated that a VFLG was successfully wrapped around the LFP particle offers a versatile way to enhanced electrochemical performance of LFP/VFLG composites. The galvanostatic discharge profiles showed an enhancing specific discharge capacity of up to 58.3% after incorporating VFLG. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests revealed that LFP/VFLG delivered small polarization, low inner resistance, high electrochemical reaction reversibility, and high lithium-ion diffusion coefficient. VFLG is a promising additive to improve the structural and electrochemical properties of the LiFePO4 cathode of lithium-ion batteries.

Effects of polyoxymethylene dimethyl ethers (PODEn) on soot characteristics in isooctane inverse diffusion flames

Polyoxymethylene dimethyl ethers (PODEn, n = 1–3) such as dimethoxymethane (DMM) and PODE3 are considered highly promising alternative fuels due to their lack of C–C bonds and significant oxygen contents. Particularly, PODE3-blends with diesel have been proven to substantially reduce soot emissions and fuel consumption rates under optimized engine conditions, although the impact on soot nanostructure properties remains unclear. This study explored the effects of DMM and PODE3 on soot characteristics in isooctane (i-octane) inverse diffusion flames, focusing on soot morphology, soot nanostructure, soot nanocrystallite, and soot spectral features. To compare soot particles behavior, DMM and PODE3 blended as the isooctane substitute (by 20% and 40% volumetric fractions) were examined with various diagnostic techniques, including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).The results showed that DMM and PODE3 contributed to a reduction in sooting tendency from i-octane flames, whereas the average soot mass decreased significantly as the PODE3 ratio increased, consistent with a higher oxygen concentration. Therefore, 20% PODE3/i-octane exhibited growing disorganized arrangements of soot particles with shorter fringes, smaller crystallite sizes, and a relatively lower degree of graphitization than 20% and 40% DMM additions. However, XPS analysis indicated that soot produced from PODE3/i-octane mixtures had less oxygen atoms because PODE3 has multiple –CH2O- chain groups that can convert oxygen into carbon monoxide with little soot production. Moreover, HRTEM, XRD, and Raman spectra analyses of 40% PODE3-doped flames confirmed an exceptional correlation with turbostratic soot structure, as manifested by the shortest fringe length, longest fringe tortuosity, more amorphous soot with the smallest crystallite dimensions, and least degree of graphitization.

Iridium nanoparticles supported on polyaniline nanotubes for peroxidase mimicking towards total antioxidant capacity assay of fruits and vegetables

In this study, a straightforward approach is presented for the first time to anchor Ir nanoparticles on the surface of uniform polyaniline (PANi) nanotubes (NTs), which can be used as an efficient peroxidase (POD)-like catalyst. The morphology and chemical structure of the PANi-Ir nanocomposite are characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffractometer (XRD), Raman and X-ray photoelectron spectroscopy (XPS) measurements. Owing to the strong interaction between Ir nanoparticles and PANi, a remarkable catalytic enhancement is achieved compared to the bare Ir black catalyst and individual PANi NTs, dominating withan electron transfer mechanism. Furthermore, an efficient colorimetric sensor for ascorbic acid (AA) is developed with a low detection limit of 1.0 μM (S/N = 3), and a total antioxidant capacity (TAC) sensing platform is also constructed for the rigorous detection and analysis of fruits and vegetables.

Revisiting the driving force inducing phase separation in PEG-phosphate aqueous biphasic systems.

Liquid phase separation using aqueous biphasic systems (ABS) is widely used in industrial processes for the extraction, separation and purification of macromolecules. Using water as the single solvent, a wide variety of solutes have been used to induce phase separation including polymers, ionic liquids or salts. For each system, polymer-polymer, polymer-ionic liquid, polymer-salt or salt-salt, different driving forces were proposed to induce phase separation. Specifically, for polymer-salt systems, a difference in solvation structure between the polymer-rich and the salt-rich was proposed, while other reports suggested that a large change in enthalpy and entropy accompanied the phase separation. Here, we reinvestigated the PEG/K2HPO4/H2O systems using a combination of liquid-phase nuclear magnetic resonance (NMR) and high-resolution Raman spectroscopies, coupled with injection microcalorimetry. Both NMR and Raman reveal a decreased water concentration in the PEG-rich phase, with nonetheless no significant differences observed for both 1H chemical shift or OH stretching vibrations. Hence, both PEG- and salt-rich phases exhibit similar water solvation properties, which is thus not the driving force for phase separation. Furthermore, NMR reveals that PEG interacts with salt ions in the PEG-rich solution, inducing a downfield shift with increasing salt concentration. Injection microcalorimetry measurements were carried out to investigate any effect due to enthalpy change during mixing. Nevertheless, these measurements indicate very small enthalpy changes when mixing PEG- and salt-rich solutions in comparison with that previously recorded for salt-salt systems or associated with mixing of two solvents. Hence, our study discards any large change of enthalpy as the origin for phase separation of PEG/K2HPO4 systems, in addition to large difference in solvation properties.

Fe-Ni porphyrin/mesoporous titania thin film electrodes: a bioinspired nanoarchitecture for photoelectrocatalysis.

Porphyrin and porphyrinoid derivatives have been extensively studied in the assembly of catalysts and sensors, seeking biomimetic and bioinspired activity. In particular, Fe and Ni porphyrins can be used for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by immobilization of these molecular catalysts on semiconductor materials. In this study, we designed a hybrid material containing a crystalline mesoporous TiO2 thin film in which the catalytic centres are Ni-porphyrin (NiP), Fe-porphyrin (FeP), and a NiP/FeP bimetallic system to assess whether the coexistence of both metalloporphyrins improves the OER activity. The obtained photoelectrodes were physicochemically and morphologically characterized through high-resolution FE-SEM images, UV-vis and Raman spectroscopies, cyclic voltammetry, and impedance measurements. The results show a differential behavior of the mono- and bimetallic porphyrin systems, where the Fe(iii) centre in FeP may increase the acidity and lower the reduction potential of the Ni2+/3+ couple when co-deposited with NiP leading to an improved photoelectrochemical water-oxidation performance. We have validated the cooperative effect of both metal complexes within this novel system, where the μ-peroxo-bridged interaction between Fe and Ni is integrated into a supramolecular heterometallic structure of porphyrins.

Enhanced LED light-driven antibacterial system with efficient charge transfer coordinated by Bi2WO6/TiO2 Z-scheme heterojunction

Designing efficient antimicrobials for the rapid disinfection of water is increasingly critical owing to the rising threat of pathogenic microorganisms. In this study, direct Z-scheme Bi2WO6/TiO2 heterojunctions were constructed using a simple in situ hydrothermal method. The optimal Bi2WO6/TiO2 Z-scheme heterojunction inhibited 98.80 % of Escherichia coli (5 × 108 CFU mL−1) at 0.35 mg mL−1 and 99.33 % of Staphylococcus aureus (5 × 108 CFU mL−1) at 0.5 mg mL−1 after 10 min of light-emitting diode (LED) light irradiation. Furthermore, in antibiotic degradation experiments, Bi2WO6/TiO2 had a 92.91 % removal rate for ciprofloxacin with a pseudo-second-order degradation rate constant (k) of 0.056 min−1 after 240 min of irradiation. The antibacterial mechanism of Bi2WO6/TiO2 was systematically investigated and confirmed to occur by LED light-driven surface activation and efficient charge transfer. The surface and interfacial structures of the Bi2WO6/TiO2 samples were studied by high-resolution transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The antibacterial mechanism was studied by controlled experiments and electron spin resonance analyses, which revealed that the primary antibacterial mechanism of Bi2WO6/TiO2 is oxidative sterilisation via generated reactive oxygen species. Time-resolved fluorescence spectroscopy was used to analyse the charge-transfer pathway in Bi2WO6/TiO2, which demonstrated that the spatial separation of the redox-active sites extended the lifetimes of the photogenerated carriers, enhancing the oxidative destruction of bacteria. This comprehensive understanding paves the way for the design of new Z-type heterojunction photocatalytic materials for antimicrobial applications.

Pd-Nanoparticle-Decorated Multilayered MoS2 Sheets for Highly Sensitive Hydrogen Sensing

In this work, efficient hydrogen gas sensors based on multilayered p-type bare MoS2 and Pd-decorated MoS2 were fabricated. MoS2 was deposited onto alumina transducers using an airbrushing technique to be used as a sensing material. Aerosol-assisted chemical vapor deposition (AACVD) was used to decorate layered MoS2 with Pd nanoparticles at 250 °C. The bare and Pd-decorated MoS2 was characterized using field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and Raman spectroscopy. The characterization results reveal the multilayered crystalline structure of MoS2 with successful Pd decoration. The size of the Pd nanoparticles ranges from 15 nm to 23 nm. Gas sensing studies reveal that a maximum response of 55% is achieved for Pd-decorated MoS2 operated at 150 °C to 100 ppm of H2, which is clearly below the explosive limit (4%) in air. The higher sensitivity due to Pd nanoparticle decoration was owed to a spillover effect. This study reveals that the sensitivity of the sensors is highly dependent on the amount of Pd decoration. Moreover, sensor responses increase slightly when exposed to 50% relative humidity (RH at 25 °C).

Green synthesised nanocopper/chitosan aerogel biocomposite as a recyclable and nonprecious catalyst for methyl orange reduction

Methyl orange (MO), which is known as a highly durable dye that is extremely toxic to humans and animals, has been processed by using various methods. In recent years, MO degradation through reduction in the presence of NaBH4 (NB) and suitable nanocatalyst have garnered considerable interest. However, the nanosized particles of the catalyst make separation from the liquid solution more difficult and limit the practical application of this technique. Herein, copper nanoparticles (CuNPs) decorated on three-dimensional chitosan aerogel (CSA) were synthesized as a reusable composite for the catalytic degradation of MO by using a green method with durian (Durio zibethinus) shell (DS) extract as a reducing agent. The Cu/CSA composite’s physicochemical properties were characterized by using X-ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Scanning electron microscopy- energy dispersive X-ray spectroscopy (SEM-EDS), high resolution Transmission electron microscopy (HR-TEM), Brunauer-Emmett-Teller (BET) and Raman spectroscopy. These modern techniques elucidated the structure and formation of copper nanoparticles on the CSA surface. The copper nanocatalysts have an average particle size of 32.6 nm, specific surface area of 55.8 m2/g, pore size of 32 Å and pore area of 0.049 cm3/g. Through the investigation of catalyst activity, the conditions of MO reduction were optimised with the 1.0 Cu/CSA sample, catalyst dose of 0.5 g/L and a molar ratio of MO/NB of 1/200. The reaction reached over 94.3% efficiency after 10 min, correlating with a kinetic rate constant of 0.163 min−1. It is worth mentioning that the optimized catalyst can be conveniently synthesized and retrieved from the reaction solution in just 5 s using a vacuum filter. Furthermore, its effectiveness remains largely unaffected even after undergoing five cycles of reuse, thus demonstrating its exceptional durability and potential in environmental remediation.

Assessing the quality of large-area monolayer graphene grown on liquid copper for size-selective ionic/molecular membrane separations

Monolayer graphene growth on liquid copper (Cu) has attracted attention due to advantages of a flat/smooth catalytic growth surface, high synthesis temperature (>1080 °C) as well as the possibility of forming graphene domains that are mobile on the liquid Cu with potential to minimize grain boundary defects and self-assemble into a continuous monolayer film. However, the quality of monolayer graphene grown on liquid copper and its suitability for size-selective ionic/molecular membrane separations has not been evaluated/studied. Here, we probe the quality of monolayer graphene grown on liquid Cu (via a metallurgical process, HSMG®) using Scanning Electron Microscope (SEM), High-resolution transmission electron microscope (HR-TEM), Raman spectroscopy and report on a facile approach to assess intrinsic sub-nanometer to nanometer-scale defects over centimeter-scale areas. We demonstrate high transfer yields of monolayer graphene (>93% coverage) from the growth substrate to polyimide track etched membrane (PITEM, pore diameter ∼200 nm) supports to form centimeter-scale atomically thin membranes. Next, we use pressure-driven transport of ethanol to probe defects > 60 nm and diffusion-driven transport of analytes (KCl ∼0.66 nm, L-Tryptophan ∼0.7–0.9 nm, Vitamin B12 ∼1–1.5 nm and Lysozyme ∼3.8–4 nm) to probe nanoscale and sub-nanometer scale defects. Diffusive transport confirms the presence of intrinsic sub-nanometer to nanometer scale defects in monolayer graphene grown on liquid Cu are no less than that in high-quality graphene synthesized via chemical vapor deposition (CVD) on solid Cu. Our work not only benchmarks quality of graphene grown on liquid copper for membrane applications but also provides fundamental insights into the origin of intrinsic defects in large-area graphene synthesized via bottom-up processes for membrane applications.

Thermal strain relaxation of GaAs overgrown on nanovoid based Ge/Si substrate

The integration of high quality III-V materials-based devices on the Si platform is considered as an enabling path towards achieving the long-standing goal of creating low-cost/high-performance devices. However, the heteroepitaxy of III-V compounds on a Si substrate generates high stress due to the difference in thermal expansion coefficient leading notably to crack generation. Here, we report on the reduction of the thermal strain of GaAs and Ge deposited on a Si substrate. The method is based on the formation of buried high density nanovoids by electrochemical etching of Ge/Si virtual substrate followed by direct epitaxial growth of Ge and GaAs. The thermal strain has been investigated for GaAs epilayer by High-Resolution X-Ray Diffraction and Raman spectroscopy showing a 30% decrease in strain. The strain reduction relies on the nanovoids-mediated thermal strain accommodation. Furthermore, photoluminescence measurements showed a 10-fold increase of the intensity, resulting in an improved optical and crystalline quality, making this virtual substrate promising for III-V based devices on Silicon.

Highly defective graphene quantum dots-doped 1T/2H-MoS2 as an efficient composite catalyst for the hydrogen evolution reaction

We present a new composite catalyst system of highly defective graphene quantum dots (HDGQDs)-doped 1T/2H-MoS2 for efficient hydrogen evolution reactions (HER). The high electrocatalytic activity, represented by an overpotential of 136.9 mV and a Tafel slope of 57.1 mV/decade, is due to improved conductivity, a larger number of active sites in 1T-MoS2 compared to that in 2H-MoS2, and additional defects introduced by HDGQDs. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were used to characterize both the 1T/2H-MoS2 and GQDs components while Fourier-transform infrared spectroscopy (FTIR) was employed to identify the functional groups on the edge and defect sites in the HDGQDs. The morphology of the composite catalyst was also examined by field emission scanning electron microscopy (FESEM). All experimental data demonstrated that each component contributes unique advantages that synergistically lead to the significantly improved electrocatalytic activity for HER in the composite catalyst system.

Reactive transport modeling for assessing coupled hydrogeochemical processes at interfaces in deep geological repositories: from the laboratory to the real world

Abstract. Deep geological repositories with a multi-barrier concept are foreseen by various countries for the disposal of high-level radioactive waste. Simulation tools for a close-to-reality description of repository evolution scenarios are required, especially to resolve the challenging task of comparing and assessing the long-term safety of different repository concepts in different host rocks within the German site-selection process. Chemical, thermal, and pressure gradients at the interfaces of the different barriers in a repository can lead to mineral dissolution and precipitation, generating non-linear responses in transport and mechanical properties of barrier materials and host rocks. Reactive transport modeling (RTM) can be applied to investigate these perturbations and processes across temporal and spatial scales to assess subsurface evolution. Nevertheless, implementing RTM at the continuum scale while accounting for pore-scale heterogeneities and geometry evolution remains a challenge. Pore-scale simulations offer the potential to capture the complex evolution of porous media over a broad range of Peclet and Damköhler numbers, and they can be utilized to improve the RTM by using upscaling methods (Prasianakis et al., 2020). In this context, we developed “lab-on-a-chip” experiments, combining time-lapse high-resolution optical microscopy and confocal Raman spectroscopy to test extended constitutive equations to classically employed Archie's law to improve the description of changes in transport properties (e.g., diffusivity) in evolving porous media in RTM. The 3D Raman tomography of the porous media combined with pore-scale modeling enabled the derivation of upscaled transport parameters. Our results highlight the importance of calibrating pore-scale models with quantitative experiments prior to simulations over a wide range of Peclet and Damköhler numbers, whose results can be further used for the derivation of upscaled modeling parameters. The derived and parameterized constitutive equations based on simple systems have been tested in reactive transport models as a sensitivity case study to evaluate uncertainties in the predictions of the evolution of real subsurface systems such as clay–cement interfaces in deep geological repositories.