In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe2. The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 1012/cm2. The exciton capture time extracted from experimental data is around ~ 1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygen-associated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides.
For over two decades, Pd has been the primary hydrogenation metal studied for reductive catalytic water treatment applications. Herein, we report that alternative platinum group metals (Rh, Ru, Pt and Ir) can exhibit substantially higher activity, wider substrate selectivity and variable pH dependence in comparison to Pd. Cross comparison of multiple metals and oxyanion substrates provides new mechanistic insights into the heterogeneous reactions. Activity differences and pH effects mainly originate from the chemical nature of individual metals. Considering the advantages in performance and cost, results support renewed investigation of alternative hydrogenation metals to advance catalytic technologies for water purification and other environmental applications.
Multilayer MoS2 possesses highly anisotropic thermal conductivities along in-plane and cross-plane directions that could hamper heat dissipation in electronics. With about 9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed about 12 times increase in the cross-plane thermal conductivity of multilayer MoS2. Our experimental and theoretical studies reveal that this drastic change arises from the greatly strengthened interlayer interaction and heavily modified phonon dispersions along cross-plane direction, with negligible contribution from electronic thermal conductivity, despite its enhancement of 4 orders of magnitude. The anisotropic thermal conductivity in the multilayer MoS2 at ambient environment becomes almost isotropic under highly compressive strain, effectively transitioning from 2D to 3D heat dissipation. This strain tuning approach also makes possible parallel tuning of structural, thermal and electrical properties, and can be extended to the whole family of 2D Van der Waals solids, down to two layer systems.
The synthesis of polymer grafted nanoparticles that are stable at high salinities and high temperature with low retention in porous media is of paramount importance for subsurface applications including electromagnetic imaging, enhanced oil recovery and environmental remediation. Herein, we present an improved approach to synthesize and purify sub-100 nm IONPs grafted with a random copolymer poly(AMPS-co-AA) (poly(2-acrylamido-3-methylpropanesulfonate-co-acrylic acid)) by means of catalyzed amide bond formation at room temperature. The improved and uniform polymer grafting of magnetic nanoparticles led to colloidal stability of IONPs at high temperature (120 °C) in API for a month. The transport behavior of the polymer grafted IONPs was investigated in crushed and in consolidated Berea sandstone. The high poly (AMPS-co-AA) polymer level on the surface (∼34%) provided electrosteric stabilization between the NPs and weak interactions of the NPs with anionic silica and sandstone surfaces. This behavior was enabled by low affinity of Ca2+ towards the highly acidic AMPS monomers thus enabling strong solvation in API brine. In crushed Berea sandstone, the retention was reduced by three fold and nine fold relative to our earlier studies, given the improvements in the grafted polymer layer. For intact core flood experiments in Berea sandstone carried out at elevated temperature (65 °C) and pressure (1000 psi net confining stress), the retention was 519 μg/g, comparable to the value for crushed Berea sandstone. Furthermore, the addition of a relatively small amount (0.1% v/v) of commercially available sacrificial polymer (e.g., HEC-10) further reduced IONP retention to 252 μg/g or 0.17 mg/m2 by blocking retentive sites
Ultralow water content carbon dioxide-in-water (C/W) foams with gas phase volume fractions (ϕ) above 0.95 (that is <0.05 water) tend to be inherently unstable given that the large capillary pressures that cause the lamellar films to thin. Herein, we demonstrate that these C/W foams may be stabilized with viscoelastic aqueous phases formed with a single zwitterionic surfactant at a concentration of only 1% (w/v) in DI water and over a wide range of salinity. Moreover, they are stable with a foam quality ϕ up to 0.98 even for temperatures up to 120 °C. The properties of aqueous viscoelastic solutions and foams containing these solutions are examined for a series of zwitterionic amidopropylcarbobetaines, R-ONHC3H6N(CH3)2CH2CO2, where R is varied from C12–14 (coco) to C18 (oleyl) to C22 (erucyl). For the surfactants with long C18 and C22 tails, the relaxation times from complex rheology indicate the presence of viscoelastic wormlike micelles over a wide range in salinity and pH, given the high surfactant packing fraction. The apparent viscosities of these ultralow water content foams reached more than 120 cP with stabilities more than 30-fold over those for foams formed with the non-viscoelastic C12–14 surfactant. At 90 °C, the foam morphology was composed of ∼35 μm diameter bubbles with a polyhedral texture. The apparent foam viscosity typically increased with ϕ and then dropped at ϕ values higher than 0.95–0.98. The Ostwald ripening rate was slower for foams with viscoelastic versus non-viscoelastic lamellae as shown by optical microscopy, as a consequence of slower lamellar drainage rates. The ability to achieve high stabilities for ultralow water content C/W foams over a wide temperature range is of interest in various technologies including polymer and materials science, CO2 enhanced oil recovery, CO2 sequestration (by greater control of the CO2flow patterns), and possibly even hydraulic fracturing with minimal use of water to reduce the requirements for wastewater disposal.
We report a new way of storing CO2 in a highly porous hydrate structure, stabilized by silica nanoparticles (NPs). Such a porous CO2 hydrate structure was generated either by cooling down NP-stabilized CO2-in-seawater foams, or by gently mixing CO2 and seawater that contains silica NPs under CO2 hydrate-generating conditions. With the highly porous structure, enhanced desalination was also achievable when the partial meltdown of CO2 hydrate was allowed
The use of helium-air mixtures to simulate the effects of elevated temperatures in aeroacoustics is plagued by the inability to match exactly the density and sound speed ratios between the jet flow and the ambient field, all the while maintaining the same gas dynamic Mach number and jet exit velocity. Real heated jet flows are typically achieved using either propane combustion in air or kerosene combustion in air, which results in the formation of carbon-dioxide and water vapor byproducts. In an effort to level the playing field between the heat simulated helium-air mixture system and the air breathing combustion system, a theoretical model is developed to isolate the effect of combustion byproducts on these aeroacoustic parameters to see if similar discrepancies arise. The motivation is to narrow the gap between laboratory and full-scale jet noise testing. Gas properties from the new combustion model are validated by laboratory measurements of a real propane combustion system as well as outputs from NASA’s Chemical Equilibrium with Applications code. The findings reveal how the additional combustion byproducts from propane combustion in air and kerosene combustion in air have a negligible effect on the parameters relevant to jet noise. Closer inspection of the helium-air mixture system demonstrates how variations in the Mach wave radiation angle at moderate pressure and temperature ratios of the nozzle is accurate to within a couple of degrees, relative to a pure heated air system. Similar accuracy is reported with the far-field intensity.
Three techniques to measure and understand the contact angle, θ, of a CO2/brine/rock system relevant to geologic carbon storage were performed with Mount Simon sandstone. Traditional sessile drop measurements of CO2/brine on the sample were conducted and a water-wet system was observed, as is expected. A novel series of measurements inside of a Mount Simon core, using a micro X-ray computed tomography imaging system with the ability to scan samples at elevated pressures, was used to examine the θ of residual bubbles of CO2. Within the sandstone core the matrix appeared to be neutrally wetting, with an average θ around 90°. A large standard deviation of θ (20.8°) within the core was also observed. To resolve this discrepancy between experimental measurements, a series of Lattice Boltzmann model simulations were performed with differing intrinsic θ values. The model results with a θ=80° were shown to match the core measurements closely, in both magnitude and variation. The small volume and complex geometry of the pore spaces that CO2 was trapped in is the most likely explanation of this discrepancy between measured values, though further work is warranted.
Polarized light is commonly used to detect optical anisotropies, such as birefringence, in tissues. This optical anisotropy is often attributed to underlying structural anisotropy in tissue, which may originate from regularly aligned collagen fibers. In these cases, the optical anisotropy, such as birefringence, is interpreted as a relative measure of the structural anisotropy of the collagen fibers. However, the relative amplitude of optical anisotropy depends on factors other than fiber orientation, and few models allow quantitative interpretation of absolute measures of true fiber orientation distribution from the optical signal. Our model uses the Mie solution to scattering of linearly polarized light from infinite cylindrical scatterers. The model is expanded to include populations of scatterers with physiologically relevant size and orientation distributions. We investigated the influences of fiber diameter, orientation distribution, and wavelength on the back-scattering signal with our computational model, and used these results to extract structural information from experimental fiber phantoms and bovine tendon. Our results demonstrated that by fitting our model to the experimental data using limited assumptions, we could extract fiber orientation distributions and diameters that were comparable to those found in scanning electron microscope images of the same fiber sample. We found a higher alignment of fibers in the bovine tendon sample, and the extracted fiber diameter was within the expected physiological range. Our model showed that the amplitude of optical anisotropy can vary widely due to factors other than the orientation distribution of fiber structures, including index of refraction, and therefore should not be taken as a sole indicator of structural anisotropy. This work highlights that the accuracy of model assumptions plays a crucial role in extracting quantitative structural information from optical anisotropy.