![]() ![]() Rashba effect in a single colloidal CsPbBr3 perovskite nanocrystal detected by magneto-optical measurements. Rashba band splitting in organohalide lead perovskites: bulk and surface effects. Rashba and Dresselhaus effects in hybrid organic–inorganic perovskites: from basics to devices. Rashba spin–orbit coupling enhanced carrier lifetime in CH 3NH 3PbI 3. Switchable S=1/2 and J=1/2 Rashba bands in ferroelectric halide perovskites. Oscillatory effects and the magnetic susceptibility of carriers in inversion layers. Observation of the “dark exciton” in CdSe quantum dots. Concerning the interpretation of complex spectra, especially the elements scandium to nickel. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices. For semiconductor nanocrystals, which are already used in lighting 17, lasers 18 and displays 19, these excitons could lead to materials with brighter emission. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster 12 than any other semiconductor nanocrystal at room 13, 14, 15, 16 and cryogenic 4 temperatures, respectively. We then apply our model to CsPbX 3 nanocrystals 11, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect 5, 6, 7, 8, 9, 10. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX 3, with X = Cl, Br or I) involves a highly emissive triplet state. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. For inorganic semiconductors, similar rules 3 predict an analogue of this triplet state known as the ‘dark exciton’ 4. For organic materials, Hund’s rules 2 state that the lowest-energy exciton is a poorly emitting triplet state. Nanostructured semiconductors emit light from electronic states known as excitons 1. ![]()
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