Epitaxial p-i-n structures grown on native GaN substrates have been fabricated and used to extract the impact ionization coefficients in GaN. The photomultiplication method has been used to experimentally determine the impact ionization coefficients; avalanche dominated breakdown is confirmed by variable-temperature breakdown measurements. To facilitate photomultiplication measurements of both electrons and holes, the structures include a thin pseudomorphic In0.07Ga0.93N layer on the cathode side of the drift layer. Illumination with 193 nm and 390 nm UV light has been performed on diodes with different intrinsic layer thicknesses. From the measured multiplication characteristics, the impact ionization coefficients of electrons (α) and holes (β) were determined for GaN over the electric field range from 2 MV/cm to 3.7 MV/cm. The results show that for transport along the c-axis, holes dominate the impact ionization process at lower electric field strengths; the impact ionization coefficient of electrons becomes comparable to that of holes (β/α<5) for electric field strengths above 3.3 MV/cm.

Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge1−xCx (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge0.998C0.002 shows a bandgap reduction supporting these results. Growth of Ge0.998C0.002 using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III–V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.

Frequency-domain near-infrared spectroscopy (FD-NIRS) provides quantitative noninvasive measurements of tissue optical absorption and scattering, as well as a safe and accurate method for characterizing tissue composition and metabolism. However, the poor scalability and high complexity of most FD-NIRS systems assembled to date have contributed to its limited clinical impact. To address these shortcomings, we present a scalable, digital-based FD-NIRS platform capable of measuring optical properties and tissue chromophore concentrations in real-time. The system provides single-channel FD-NIRS amplitude/phase, optical property, and chromophore data at a maximum display rate of 36.6 kHz, 17.9 kHz, and 10.2 kHz, respectively, and can be scaled to multiple channels as well as integrated into a handheld format. The entire system is enabled by several innovations including an ultra-high-speed k-nearest neighbor lookup table method (maximum of 250,000 inversions/s for a large 2500x700 table of absorption and reduced scattering coefficients), embedded FPGA and CPU high-speed co-processing, and high-speed data transfer (due to on-board processing). We show that our 6-wavelength, broad modulation bandwidth (1-400 MHz) system can be used to perform 2D high-density spatial mapping of optical properties and high speed quantification of hemodynamics.

No abstract

Two-dimensional molybdenum disulfide (MoS 2 ) is emerging as a catalyst for energy and environmental applications. Recent studies have suggested the stability of MoS 2 is questionable when exposed to oxidizing conditions found in water and air. In this study, the aqueous stability of 2H-and 1T-MoS 2 and 2H-MoS 2 protected with a carbon shell was evaluated in the presence of model oxidants (O 2 , NO 2 À , BrO 3 À ). The MoS 2 electrocatalytic performance and stability was characterized using linear sweep voltammetry and chronoamperometry. In the presence of dissolved oxygen (DO) only, 2H-and 1T-MoS 2 were relatively stable, with SO 4 2À formation of only 2.5% and 3.1%, respectively. The presence of NO 2 À resulted in drastically different results, with SO 4 2À formations of 11% and 14% for 2H-and 1T-MoS 2 , respectively. When NO 2 À was present without DO, the 2H-and 1T-MoS 2 remained relatively stable with SO 4 2À formations of only 4.2% and 3.3%, respectively. Similar results were observed when BrO 3 À was used as an oxidant. Collectively, these results indicate that the oxidation of 2H-and 1T-MoS 2 can be severe in the presence of these aqueous oxidants but that DO is also required. To investigate the ability of a capping agent to protect the MoS 2 from oxidation, a carbon shell was added to 2H-MoS 2 . In a batch suspension in the presence of DO and NO 2 À , the 2H-MoS 2 with the carbon shell exhibited good stability with no oxidation observed. The activity of 2H-MoS 2 electrodes was then evaluated for the hydrogen evolution reaction by a Tafel analysis. The carbon shell improved the activity of 2H-MoS 2 with a decrease in the Tafel slope from 451 to 371 mV dec À1 . The electrode stability, characterized by chronopotentiometry, was also enhanced for the 2H-MoS 2 coated with a carbon shell, with no marked degradation in current density observed over the reaction period. Because of the instability exhibited by unprotected MoS 2 , it will only be a useful catalyst if measures are taken to protect the surface from oxidation. Further, given the propensity of MoS 2 to undergo oxidation in aqueous solutions, caution should be used when describing it as a true catalyst for reduction reactions (e.g., H 2 evolution), unless proven otherwise. rsc.li/rsc-advances 9324 | RSC Adv., 2020, 10, 9324-9334This journal is Fig. 8 Chronoamperometry of 2H-MoS 2 and 2H-MoS 2 /C0.1 electrodes in the absence and presence of NO 2 À (7.14 mM). The applied potential was À0.5 V vs. RHE. Samples were degassed with N 2 prior to measurement. The observed noise in current density is due to effects from stirring.9332 | RSC Adv., 2020, 10, 9324-9334This journal is