The electronic structure and phase stability of MgO, ZnO, CdO, and related alloys in the rocksalt ͑B1͒, zincblende ͑B3͒, and wurtzite ͑B4͒ crystal structures were examined within first-principles band structure theory; the thermodynamically stable phases are reproduced for each material. The band alignment and bandgap deformation potentials were analyzed, showing an increase in the valence band maximum from Mg to Zn to Cd. Ternary alloy formation was explored through application of the special quasirandom structure method. The B1 structure is stable over all ͑Mg,Cd͒O compositions, as expected from the preferences of the binary oxides. The ͑Mg,Zn͒O alloy undergoes a tetrahedral to octahedral transition above 34% Mg content, in agreement with experiment. For ͑Zn,Cd͒O, a transition is predicted above 62% Cd content. These results imply that band-gap manipulation of ZnO from alloying with Mg ͑Cd͒ will be limited to 4.0 eV ͑1.6 eV͒, while preserving the tetrahedral coordination of the host.
The efficient single-photon upconversion photoluminescence (UCPL) feature of lead halide perovskite semiconductors makes it promising for developing laser cooling devices. This is an attractive potential application, but the underlying physics still remains unclear so far. By using the all-inorganic CsPbX 3 (X = Br, I) nanocrystal samples, this phenomenon was investigated by photoluminescence (PL) and timeresolved PL under different temperatures and various excitation conditions. A broad emission band located at the low-energy side of the free exciton (FE) peak was detected and deduced to be from the self-trapped exciton (STE). The lifetime of STE emission was found to be 171 ns at 10 K, much longer than that of FE. The UCPL phenomenon was then attributed to thermal activation of transformation from STEs to FEs, and the energy barrier was derived to be 103.7 meV for CsPbBr 3 and 45.2 meV for CsPb(Br/I) 3 , respectively. The transformation also can be seen from the fluorescence decay processes.
Droplet impact dynamics is vital to the understanding of several phase-change and heat-transfer phenomena. This work examines the role of substrate elasticity on the spreading and retraction behavior of water droplets impacting flat and textured superhydrophobic substrates. Experiments reveal that droplet retraction on flat surfaces decreases with decreasing substrate elasticity. This trend is confirmed through a careful measurement of droplet impact dynamics on multiple PDMS surfaces with varying elastic moduli and comparison with impact dynamics on hard silicon surfaces. These findings reveal that surfaces tend to become more wettable upon droplet impact as the elastic modulus is decreased. First-order analyses are developed to explain this reduced retraction in terms of increased viscoelastic dissipation on soft substrates. Interestingly, superhydrophobic surfaces display substrate-elasticity-invariant impact dynamics. These findings are critical when designing polymeric surfaces for fluid-surface interaction applications.
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