The solar response ability and low-cost fabrication of the photoanode are important factors for the effective output of the photoelectrochemical system. Modification of the photoanode by which its ability to absorb irradiation can be manipulated has gained tremendous attention. Here, we demonstrated the MoSe 2 , WSe 2 , and MoSe 2 /WSe 2 nanocrystal thin films prepared by the liquid-phase exfoliated and electrophoresis methods. Atomic force microscopy and high-resolution transmission electron microscopy show that the liquid exfoliated nanocrystals have a few layered dimensions with good crystallinity. Scanning electron microscopy demonstrated uniform distribution and randomly oriented nanocrystals, having a homogeneous shape and size. X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectra confirm the equal contribution of MoSe 2 and WSe 2 nanocrystals in the formation of the MoSe 2 /WSe 2 heterojunction. Because of superior absorption of MoSe 2 /WSe 2 heterojunction in the visible region and type-II heterojunction band alignment, in situ measurement of heterojunction electrode shows almost 1.5 times incident photo-to-current conversion efficiency and photoresponsivity in comparison to individual material electrodes. Our result clearly indicates the influence of heterojunction formation between liquid exfoliated nanocrystals on effective separation of photogenerated exciton and enhances charge carrier transfer, which leads to the improvement in photoelectrochemical performance. Liquid exfoliated nanosheet-based heterojunction is attractive as efficient photoanodes for the photoelectrochemical systems.
Elements with low first ionization potential (FIP) are known to be 3–4 times more abundant in active region loops of the solar corona than in the photosphere. There have been observations suggesting that this observed “FIP bias” may be different in other parts of the solar corona and such observations are thus important in understanding the underlying mechanism. The Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission carried out spectroscopic observations of the Sun in soft X-rays during the 2019–2020 solar minimum, considered to be the quietest solar minimum of the past century. These observations provided a unique opportunity to study soft X-ray spectra of the quiescent solar corona in the absence of any active regions. By modeling high-resolution broadband X-ray spectra from XSM, we estimate the temperature and emission measure during periods of possibly the lowest solar X-ray intensity. We find that the derived parameters remain nearly constant over time with a temperature around 2 MK, suggesting the emission is dominated by X-ray bright points. We also obtain the abundances of Mg, Al, and Si relative to H, and find that the FIP bias is ∼2, lower than the values observed in active regions.
Solar flares, with energies ranging over several orders of magnitude, result from impulsive release of energy due to magnetic reconnection in the corona. Barring a handful, almost all microflares observed in X-rays are associated with the solar active regions. Here we present, for the first time, a comprehensive analysis of a large sample of quiet-Sun microflares observed in soft X-rays by the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission during the 2019–2020 solar minimum. A total of 98 microflares having peak flux below GOES A-level were observed by the XSM during observations spanning 76 days. By using the derived plasma temperature and emission measure of these events obtained by fitting the XSM spectra along with volume estimates from concurrent imaging observations in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly, we estimated their thermal energies to be ranging from 3 × 1026 to 6 × 1027 erg. We present the frequency distribution of the quiet-Sun microflares with energy and discuss the implications of these observations of small-scale magnetic reconnection events outside active regions on coronal heating.
2-D
transition metal dichalcogenide (TMDC)-based heterostructures
are promising active materials for high-performance optoelectronic
devices. The low-cost, large-area, and high-quality fabrication of
TMDC heterojunctions is essential for the efficient output of the
device. Here, we demonstrate thin films of MoSe2–WSe2 nanocrystals deposited on a silicon substrate for enhanced
photodetection. A MoSe2–WSe2 film, deposited
by the electrophoretic deposition method, is initially transferred
on the water surface and then prudently transferred on the p-Si (100)
substrate. Scanning electron microscopy reveals the continuous and
compact distribution of assembled nanocrystals with no pinhole. Energy-dispersive
analysis of X-ray confirms the presence of MoSe2 and WSe2 in the transferred heterojunction film. The MoSe2–WSe2/p-Si fabricated heterojunction achieves a
peak responsivity and external quantum efficiency of 336 mAW–1 and 80% (520 nm, 0.122 mW/cm2), respectively, which are
∼4 times higher in magnitude than those of pristine TMDC/Si
fabricated heterojunctions. The enhanced photoresponse behavior is
attributed to the superior absorbance in the visible region and type-II
band alignment between MoSe2 and WSe2 nanocrystals,
which facilitates improved generation and separation of charge carriers.
Further, the photoresponse of MoSe2–WSe2/Si heterojunction is recorded in the temperature range of 45–300
K. The excellent heterojunction characteristic and photoresponse behavior
of liquid exfoliated TMDC nanocrystals are the future gateways of
highly efficient hybrid optoelectronic devices.
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