| 1. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
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Using low-level currents measured during lightning events to estimate upward leader properties
- 2018
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Ingår i: 34th International Conference on Lightning Protection, ICLP 2018. - : Institute of Electrical and Electronics Engineers Inc.. - 9781538666357
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Konferensbidrag (refereegranskat)abstract
- Low-level currents measured prior to return strokes can potentially provide information about the properties of upward leaders during lightning flashes. However, these currents need to be properly analysed and interpreted in order to be useful for evaluating upward connecting leaders. In this paper, low-level currents measured before return strokes in two lightning events to two structures in Brazil are analysed and interpreted as case studies. The discharge current estimated from one of these events is used as input to a detailed thermohydrodynamic model with an extensive kinetic scheme for N2/O2 mixtures. The model allows the evaluation of the physical and chemical properties of upward connecting leaders. Estimates of the temperature, mass density, electric field and radius of the channel are presented for an upward connecting leader propagating in a lightning event. In addition, estimates of the axial density of electrons, ions and neutral particles (including NO and NO2) are also reported.
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| 2. |
- Liu, Lige, et al.
(författare)
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Cation effect on excitons in perovskite nanocrystals from single-dot photoluminescence of CH3NH3PbI3
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:19
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Tidskriftsartikel (refereegranskat)abstract
- The single-dot photoluminescence properties of similar to 7 nm perovskite MAPbI(3) (MA = CH3NH3+) nanocrystals (NCs) were investigated in the 5-295 K temperature range both in spectral and time domains. Repeatable single-dot measurements were facilitated by the use of a protective polymer, which stabilized the NCs. Temperature-induced phase transition and exciton-phonon interactions were revealed as well as the exciton fine structure. A pronounced spectral jump of the emission peak at 140-160 K, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of similar to 0.6 meV was measured, which is the narrowest ever recorded for this perovskite material system. A similar to 4.0 meV phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of MA(+) leads to broader spectra than for Cs+ or FA(+) containing perovskite NCs. It is attributed to higher polarity of this cation, thus it is more susceptible to spectral diffusion, which is clearly observed here. Photoluminescence decay measurements indicated that the recombination from the lowest energy state of the emission level manifold is partially forbidden. This is opposite to Cs+ cation NCs, highlighting the central role of the positive ion in the exchange interaction in perovskites. Finally, delayed luminescence was found to govern the recombination dynamics below room temperature, suggesting an involvement of trap sites for the orthorhombic phase. The reported photophysics of a quantum-confined exciton in this material, which is of interest for various light-converting applications, clarifies the role of the cation in perovskite nanocrystal optical properties.
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| 3. |
- Liu, Lige, et al.
(författare)
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Photodegradation of Organometal Hybrid Perovskite Nanocrystals : Clarifying the Role of Oxygen by Single-Dot Photoluminescence
- 2019
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Ingår i: The Journal of Physical Chemistry Letters. - : AMER CHEMICAL SOC. - 1948-7185. ; 10:4, s. 864-869
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Tidskriftsartikel (refereegranskat)abstract
- Photostability has been a major issue for perovskite materials. Understanding the photodegradation mechanism and suppressing it are of central importance for applications. By investigating single-dot photoluminescence spectra and the lifetime of MAPbX(3) (MA = CH3NH3+, X = Br, I) nanocrystals with quantum confinement under different conditions, we identified two separate pathways in the photodegradation process. The first is the oxygen-assisted light-induced etching process (photochemistry). The second is the light-driven slow charge-trapping process (photophysics), taking place even in oxygen-free environment. We clarified the role of oxygen in the photodegradation process and show how the photoinduced etching can be successfully suppressed by OSTE polymer, preventing an oxygen-assisted reaction.
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| 4. |
- Liu, Lige, et al.
(författare)
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Size-Dependent Phase Transition in Perovskite Nanocrystals
- 2019
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 10:18, s. 5451-5457
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Tidskriftsartikel (refereegranskat)abstract
- Organic-inorganic hybrid perovskites simultaneously possess strong spin- orbit coupling (SOC) and structure inversion asymmetry, establishing a Rashba effect to influence light emission and photovoltaics. Here, we use mechanical bending as a convenient approach to investigate the Rashba effect through SOC in perovskite (MAPbI(3-x)Cl(x)) films by elastically deforming grains. It is observed that applying a concave bending can broaden the line shape of the magnetophotocurrent, increasing the internal magnetic parameter B-0 from 121 to 205 mT, which indicates an enhancement on SOC. Interestingly, the PL lifetime is found to be enlarged from 9.9 to 14.8 ns under this bending, which suggests that introducing compressive strain can essentially increase the Rashba effect through SOC, leading to an increase upon indirect band transition. Furthermore, the PL peak associated with the Rashba effect is shifted from 776 to 780 nm under this mechanical bending. Therefore, mechanical bending provides a convenient experimental method to approach the Rashba effect in hybrid perovskites.
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