The intensity‐scan (I‐scan) technique to study the polarization‐dependent, nonlinear processes in exfoliated bulk ReS2 is utilized. The polarization‐dependent reflection and transmission of ReS2, from which the absorption coefficients are extracted using the transfer matrix method, are measured. Absorption coefficients under high laser peak power show a transition from saturable absorption (SA) to reverse saturable absorption when rotating the laser polarization with respect to the b‐axis. It is found that SA and excited‐state absorption (ESA) contribute to the nonlinear optical processes. Both the SA and ESA show strong dependence on the polarization angle, which is attributed to the anisotropic optical transition probability and electronic band structure in ReS2. The anisotropic nonlinear optical properties of ReS2 may find applications as saturable absorbers in lasers and optical modulators.
Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides (TMDs). Here, the phenomenon of oxygen impurities trapping photoexcited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect-capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First-principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in TMDs, and facilitate defect engineering in relevant materials and device applications.
Optical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) By adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1).
A high electrical and thermal conductivity coupled with low costs make copper (Cu) an enticing alternative to aluminum for fabrication of interconnects in packaging applications. To tap into the benefits of the ever-reducing size of transistors, it is required to increase the input/output (I/O) pin count on electronic chips and thus minimize the size of chip to board interconnects. Laser sintering of Cu nanoparticle (NP) inks can serve as a promising process for developing these micron sized, 3D interconnect structures. However, the exact processing windows for Cu NP sintering are not well known. Therefore, this paper presents an extensive experimental investigation of the sintering processing window with different lasers including femtosecond (fs), nanosecond (ns) and continuous-wave (CW) lasers. The dependence of the processing window on Cu layer thicknesses and laser exposure durations has also been investigated. A simplified model to estimate optimum laser sintering windows for Cu NPs using pulsed lasers is presented and the predicted estimates are compared against the experimental results. Given the simplicity of the model, it is shown to provide good estimates for fluence required for the onset of sintering and the processing window for good sintering of Cu NPs.
There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.
In this work, we experimentally demonstrate metasurface-enhanced photoresponse in organic photodetectors. We have designed and integrated a metasurface with broadband functionality into an organic photodetector, with the goal of significantly increasing the absorption of light and generated photocurrent from 560 up to 690 nm. We discuss how the metasurface can be integrated with the fabrication of an organic photodiode. Our results show large gains in responsivity from 1.5x to 2X between 560 and 690 nm.
The environmental impacts of a typical hydraulic fracturing operation for shale gas recovery were evaluated using life cycle assessment, with energy demands for well drilling and fracturing determined from GHGfrack model. Dominant environmental impacts stem from well construction, which are >63% in all categories (e.g., global warming and eutrophication), and mainly due to diesel fuel combustion and steel production. The relative impacts related to water use (i.e., fracturing fluid components, water/wastewater transportation, and wastewater disposal) are relatively small, ranging from 5 to 22% of total impacts in all categories; freshwater consumption for fracturing is also a small fraction of available water resources for the shale play considered. The impacts of replacing slickwater with CO2 or CH4-foam fracturing fluid (≤10 vol % water) were evaluated; total impacts decrease <12%, and relative impacts related to water use decrease to 2–9% of total impacts. Hence, switching to a foam-based fracturing fluid can substantially decrease water-related impacts (>60%) but has only marginal effects on total environmental impacts. Changes in lateral well length, produced to fresh-water ratios, fracturing fluid composition, and LCA control volume do not change these findings. More benefits could potentially be realized by considering water versus foam-related impacts of ecological health and energy production.