There is a high prevalence of nonalcoholic fatty liver among certain population in Shanghai, to which overweight and hyperlipidemia are closely relevant.
By combining the electron−phonon coupling effect and the static coupling formalism, we calculate, through the firstprinciples methods, the carrier capture cross sections of the three possible nonradiative recombination center (NRRC) defects in Cu 2 ZnSnS 4 . These values are currently unavailable but critical for understanding the limiting factors of the minority carrier lifetime and simulating the photovoltaic devices. We show that the cross sections for Sn Zn 2+ capturing one electron (a (+2/+1) transition) and for [Cu Zn −Sn Zn ] + capturing one electron (a (+1/0) transition) are both very large, whereas for Sn Zn + capturing one electron (a (+1/0) transition) is much smaller by several orders of magnitude. The minority carrier lifetime will be limited to below 1 ns if the concentrations of Sn Zn 2+ and [Cu Zn −Sn Zn ] + are higher than 10 15 cm −3 , so they are effective NRRCs, whereas the lifetime can be as long as 10 μs with the same concentration of Sn Zn + , so Sn Zn + is a noneffective NRRC. The phonon mode analysis shows that the cross section is strongly correlated with the vibration mode of Sn−S bonds around the defects and its coupling with the localized wavefunction on the defect state. Sn Zn 2+ and [Cu Zn −Sn Zn ] + have a short and strong Sn−S bond with a highfrequency vibration mode, and these two defects undergo a large structural distortion after capturing an electron, which decreases the barrier for carrier capture and thus produces a large cross section. In contrast, Sn Zn + has a softer Sn−S vibration mode and thus much higher barrier for electron capture. Our calculations not only identify two effective NRRCs, which provide the mechanism behind why the Cu-poor, Zn-rich, Sn-poor growth condition, were widely adopted for fabricating high-efficiency Cu 2 ZnSnS 4 solar cells but also show that a very large difference can exist in the carrier capture cross sections for the same defect in different charge states (Sn Zn 2+ vs Sn Zn + ). We propose that the deep-level defects may have large carrier capture cross sections if they are surrounded by strong bonds and undergo considerable structural relaxations after capturing a carrier, which can be used as an empirical criterion for the quick identification of effective NRRCs.
Molecular fluorescence blinking provides a simple and attractive way to achieve super-resolution localization via conventional fluorescence microscopy. However, success in super-resolution imaging relies heavily on their blinking characteristics. We here report easily prepared and photostable nanoparticles, carbon dots (CDs), with desirable fluorescence blinking for high-density super-resolution imaging. The CDs exhibit a low duty cycle (∼0.003) and high photon output (∼8000) per switching event, as well as show much higher resistance to photobleaching than Alexa 647 or Cy5 typically used in single molecule localization microscopy. The stable blinking of CDs allows to perform high-density localization imaging at a resolution of 25 nm by sequentially recording the particle positions. The CD-based super-resolution imaging is further demonstrated by rendering CD-stained tubular peptide self-assemblies, CD-packed clusters with well-defined patterns, and CD-stained microtubules in a cell. Furthermore, this method has been validated as a valuable tool to detect the clustering and distribution of protein receptors in the plasma membrane that are not discerned with normal fluorescence imaging.
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