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.

The adsorption structures and mechanisms of water adsorption on ZnO(0001) and GaN(0001) surface are investigated by using the first-principles methods. It is found that the stable adsorption structure at full monolayer (ML) coverage is (2 × 1) reconstructed. A (2 × 1) molecular adsorption is definite for ZnO, and a (2 × 1) dissociative adsorption is also possible for GaN. For these structures the hydrogen bonds between adsorbates are significant besides the covalent interaction with substrate. For the coverage below 0.5 ML for GaN and 0.25 ML for ZnO, the individually adsorbed H2O can easily decompose to OH and H. Both covalent and electrostatic attractions contribute to the stability of dissociative adsorption. For the coverage between the above two cases, molecular adsorption is found to be stable in theory, but the real structure may be greatly dependent on the chemical condition. These results give a detailed description of the interaction between the first water adlayer and ZnO(GaN)(0001) surface.

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The effect of an electric field on the electronic properties of a MoS2 monolayer between two graphene sheets (G/MoS2/G) is investigated within the framework of density functional theory. We show that the positive and negative electronic field applied in the direction perpendicular to the G/MoS2/G superlattice significantly modifies the electronic structure of the whole system, which can allow to control the values of the energy gap. It is shown that the energy dispersions are nearly linear in the vicinity of the Fermi level with and without external field. We elucidate the mechanism for the gap tuning by examining the projected density of states of C atoms and charge redistribution within graphene and charge transfer between graphene and MoS2 layers driven by the electric field. These findings are a useful complement to experimental studies of the G/MoS2/G system and provide a theoretical explanation for the extraordinary performance of this superlattice for fast speed and high on-off switching transistor.