Excitation-power dynamics of near-band-edge photoluminescence (PL) peak position in In x Ga 1−x N / GaN multiple quantum wells ͑x ϳ 0.15͒ was analyzed as a function of well width. The analysis was based on energy reference provided by photoreflectance (PR) spectra. The difference in spectral position of the PR feature and low-excitation PL band (the Stokes Shift) revealed carrier localization energy, which exhibited a remarkable sensitivity to the well width, increasing from 75 meV in 2 nm wells to about 250 meV in 4 nm wells. Meanwhile collating of the PR data with the flat-band model for the optical transition energy in quantum wells rendered a relatively weak ͑0.5 MV/ cm͒ built-in piezoelectric field. The blueshift of the PL peak position with increasing photoexcitation power density was shown to be in qualitative agreement with the model of filling of the band-tail states with some contribution from screening of built-in field in the thickest ͑4 nm͒ wells. Increased incident photon energy resulted in an additional blueshift of the PL peak, which was explained by a nonthermalized distribution of localized carriers and/or carrier localization in the interface region. Our results are consistent with a concept of emission from partially relaxed large In-rich regions with internal band potential fluctuations, which are enhanced with increasing the growth time.
Aluminum alloy materials, with their light-weight and low-cost characteristics, are considered to be some of the most suitable materials for use as bipolar plates (BPPs). In this study, a chromiumcarbon/nickel-phosphorus (Cr-C/Ni-P) bilayer coating is deposited on aluminum alloy 5052 (AA5052) to improve the corrosion resistance and interfacial contact resistance. An electroless Ni-P layer is initially deposited on the AA5052 to form an undercoat, onto which a Cr-C coating is subsequently deposited by electroplating from a trivalent chromium bath. The surface morphology and chemical composition of the prepared coatings were investigated using scanning electron microscopy (SEM) and electron probe X-ray micro-analyzer (EPMA) techniques. The surface morphology of the Cr-C coating that formed with a plating time of 10 min is smooth and crack-and pinhole-free; however, cracks and pinholes are clearly observed in deposits that have been plated for a longer plating time. Therefore, the potentiodynamic test results indicate that the coating prepared at 10 A dm -2 and 10 min possesses the best corrosion resistance (i corr = 6.7×10). The contact resistance of the Cr-C coating plated at 10 min is the lowest (8.2 mΩ·cm 2 ). The aluminum BPPs that have a Cr-C/Ni-P bilayer coating exhibits great potential for PEMFC applications.
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