Fast-moving stars around an intermediate-mass black hole in ω Centauri.

Black holes have been found over a wide range of masses, from stellar remnants with masses of 5-150 solar masses (M☉), to those found at the centres of galaxies with M > 105M☉. However, only a few debated candidate black holes exist between 150M☉ and 105M☉. Determining the population of these intermediate-mass black holes is an important step towards understanding supermassive black hole formation in the early universe1,2. Several studies have claimed the detection of a central black hole in ω Centauri, the most massive globular cluster of the Milky Way3-5. However, these studies have been questioned because of the possible mass contribution of stellar mass black holes, their sensitivity to the cluster centre and the lack of fast-moving stars above the escape velocity6-9. Here we report the observations of seven fast-moving stars in the central 3 arcsec (0.08 pc) of ω Centauri. The velocities of the fast-moving stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can be explained only by being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit of the black hole mass of about 8,200M☉, making this a good case for an intermediate-mass black hole in the local universe.

Recent Progress on Perovskite Surfaces and Interfaces in Optoelectronic Devices.

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.

Multiple Self-Trapped Emissions in the Lead-Free Halide Cs3Cu2I5.

Low-dimensional copper halides with high luminance have attracted increasing interest as heavy-metal-free light emitters. However, the optical mechanisms underpinning their excellent luminescence remain underexplored. Here, we report multiple self-trapped emissions in Cs3Cu2I5. Power-dependent photoluminescence spectra reveal the appearance of multiple self-trapped emission peaks with increasing excitation power, and this emission behavior is explored across a temperature range of 80-420 K. The zero-dimensional structure and soft crystal lattice contribute to the multiple self-trapped emissions in Cs3Cu2I5: this explains the origin of the broad emission and the luminescence mechanism in Cs3Cu2I5 and will assist in improving our understanding of the optical properties of other metal halides. We incorporate the Cs3Cu2I5 in light-emitting diodes that achieve a peak luminance of 140 cd/m2 and an external quantum efficiency of 0.27%.